Notes at the start:
Background: In the TV series Teen Titans the producers decided to give Raven a three word chant. She recites it when she has to do strong magic in combat. She recites it when she is meditating. These are obviously very different situations. This gives rise to the question, "Just what are these words?"
The first word is Azarath, the name of her home. The other two words, Metrion and Zinthos, have no canon meaning. The writers just made them up without a thought as to what they might mean. They're left as an exercise to the fanfiction writer.
One fanfic writer's version has attained deuterocanonical status to the point that when I posted this to Fanfiction.net the immediate response was someone "correcting" me with that version as if it were unalterable truth.
This conversation will eventually take place in With great power comes .?. but I haven't even gotten to the point where Raven knows that Lisa was Terra. At the moment in that story, Raven got a bad read and is under the impression that Lisa isn't Terra.
"What do 'Metrion' and 'Zinthos' mean?" Lisa asked.
"How long have you been wondering that?" Raven asked in return.
"Ever since I learned that Azarath was the name of a place rather than the first word of a magic spell," Lisa said.
"It actually is a spell," Raven said. "Spells have to come from somewhere, after all, and I made that one. Quite by accident, I might add."
"You made a spell accidentally?"
"It was originally just an aid in using my power," Raven said. "Through repetition it became something more. Something else. I'd never be able to use it to center myself and meditate if it still produced the original effect."
"So .?." Lisa asked.
"My power is controlled by my emotions," Raven said, "the stronger the emotion, the stronger the power. But if I give in to negative emotions I lose control."
"I can relate."
Raven nodded. "To use my powers while staying in control I made a mantra that would summon strong positive emotions."
"Thus your home," Lisa said. "I'm only beginning to understand what that's like. To to have one, I mean."
Raven felt a pang of guilt. If Terra had been raised by a religious order of pacifists who taught her meditation, and Raven had been the one left to fend for herself, never knowing stability, never able to control her powers until Slade came with an offer to help . . .
It wasn't that Terra's actions were excusable, but they were understandable. Raven had been right to be suspicious of Terra's new control, but she could have shared her own means of control with Terra. If she had, maybe Terra wouldn't have . . .
Raven put those thoughts aside. It was too late for any of that. This girl wasn't even Terra anymore. She was Lisa now. Someone with a clean slate, a family, and a home.
"I'm happy for you," Raven said. It was a sign of how much had changed since her father was defeated that she could say that with the accompanying emotion free to control her tone, her expression, and so much more.
"Metrion is the name of a spell. It was the first major spell I ever cast. It felt like I spent forever researching it, gathering ingredients, making sure I understood the nuances of it. I almost failed, but when it was done it changed everything for me. I can't imagine my life if I hadn't cast it, and I don't want to either."
"What did it do?" Lisa asked.
"It made me more . . . me," Raven said. "Made the outside match the inside. More like my mother, less like my father."
"Were you red or something before?" Lisa asked.
"Nothing like that," Raven said. She felt herself smiling slightly.
"I don't actually know what your father looked liked," Lisa said.
"Well . . . you were right about him being red," Raven said. "But how I was before . . . it's not something I talk about."
"Oh," Lisa said. "Ok. I didn't mean to pry. Sorry if I made you-"
"It's fine," Raven said. "I hope your curiosity is sated."
"It is," Lisa said. "For that word at least. I tried to look it up, by the way, and things said Metrion was your real name but I couldn't track down the source."
"Guess," Raven said.
"Fan stuff?" Lisa asked.
"It's more than just fans," Raven said, "anyone who wants to write a story set in Jump City kind of has to include the Titans. Not to would be like writing a story set on earth in which you edited the sun out of the solar system. It wouldn't make sense. When they don't know but they want to delve, they have to make things up.
"It doesn't bother me that they think that," Raven said, "but Raven is my real name."
"I know; we talked about it," Lisa said. "What about the other fan stuff?"
"You mean the fact that I'm apparently dating Starfire?" Raven asked. Then she resumed her classic deadpan: "Wait, no. Robin. Or was it Cyborg? Definitely lots and lots of Beast Boy. I'm a regular Casanova."
"Jinx too," Lisa said, "though that kind of feels like less of a stretch."
"Why?" Raven asked.
"More common ground," Lisa said.
"I don't see it," Raven said.
"I didn't bring it up to talk about your many and varied fictional romantic escapades, though," she said. "I was wondering how you felt about it in general. People writing stories about you and all."
"I generally steer clear of it," Raven said, "but from what I've seen some is good, some is bad, some is positive, some is bashing Titans the author doesn't like, some is cathartic, some is art, some is drivel, and so forth. About what you'd expect."
"Ok," Lisa said, "what about Zinthos?"
"My first love," Raven said. "All romance; nothing physical. I hadn't even experienced physical attraction yet. We were young, and adults thought we were just playing, but the love was real, and the feeling was so powerful that sometimes I worried about losing control."
"But positive so it didn't risk you turning tentacley," Lisa said.
"Yes," Raven said. "So that was the mantra I used to be able to use my powers, then only something I considered doing for self defense, without giving into darker emotions. Over time it transformed into a personal spell. A very specific one. The effect is basically to let me do something tied to my emotions while avoiding the darkness. The upside is that it's more powerful than a mere mantra and can be used in more situations. The downside is that now that it is a spell, being interrupted before I finish saying 'Zinthos' can be a problem."
"Meditation is tied to your emotions?"
"Meditation is about my emotions," Raven said. "To control my powers I need to control my emotions so every day I meditate to face all of my emotions and make sure none of them can overpower me, slip out, bring me to a place I can't afford to be, or anything else that could lead to catastrophe."
"I should have asked you to teach me."
"I should have offered," Raven said. "It doesn't always work; sometimes I have to resort to more extreme measures, and even then I've lost control. Besides, it's all in the past since you've found your own control now."
"Are you sure?" Lisa asked.
"You're in a high school, a high stress environment if ever there were one, and there hasn't been so much as a tremor," Raven said. "I looked it up because I thought maybe chasing earthquakes would help me find you. You didn't even set off any of the tiny ones that are too minor for human beings to feel."
"That makes me feel better," Lisa said.
Mentioning that there's a theory "Metrion" is Raven's real name instead of "Raven", is paying homage to the the most widely accepted fan fic version of the words meaning. In that version Zinthos is Raven's childhood pet raven who, presumably, was very important to her given that in that version of fanon she named herself after Zinthos' species.
Also it just seems like there'd be fanfic in-universe so I figured I'd have that come up.
"Metrion" being the name of a spell that was about Raven's outside matching her inside and whatnot is very much inspired by a fic by Shadokin called Makings of a Family, in which Raven is a trans girl and this fact is met by acceptance and kindness when it is learned.
So, yeah, my version of what Metrion and Zinthos mean.