The AI companion glossary: every term, plainly explained

The AI companion world runs on jargon borrowed from machine learning, fan-fiction culture, and game design all at once. Here is every term you will actually encounter, defined plainly, with the honest note on why each one matters when you are choosing or using a platform.

The machinery

LLM (large language model). The text engine underneath every AI companion. It has no identity and no memory of its own — everything companion-like is built on top.

Context window. The fixed amount of text a model can consider at once. When a conversation outgrows it, the oldest content falls out — the root cause of companions "forgetting." Platform memory features exist to work around this ceiling; our memory guide explains the architectures.

System prompt / character definition. The hidden instructions that turn a generic model into a specific character: who they are, how they speak, what they must never do. Its quality is why two characters on the same model can feel worlds apart.

Temperature. A setting controlling how predictable or adventurous replies are. Low is precise and repetitive; high is creative and erratic. Platforms often expose it as modes like "precise" and "creative."

Memory and persistence

Session memory. Recall limited to the current conversation. The companion equivalent of a goldfish, and the default on cheaper platforms.

Persistent memory. Anything retained across sessions. Comes in two flavors: hidden summaries the platform keeps invisibly, and user-approved memory — Lusto's model — where remembered moments are visible artifacts you approve, edit, and delete.

World state. The persistent condition of the story itself: location, time, open threads, what has happened. Distinct from memory-about-you; both must persist for a story to truly resume.

Relationship state. Tracked values (on Lusto: warmth, trust, tension, chapter) that make closeness something earned and kept rather than re-improvised each session.

Checkpoint / branch. A saved story moment, and a separate timeline resumed from one. Together they let you explore alternate paths without destroying the original — see branching stories.

The craft vocabulary

Persona. The authored version of you in a story — name, background, boundaries — that the character responds to. The single highest-leverage feature most users never touch; see the persona guide.

Roleplay (RP). Conversation written as scenes — actions, dialogue, setting — rather than plain chat. Asterisk actions (*sets the cup down*) mark what you do versus what you say.

Slow burn. A story whose tension builds deliberately over many scenes. Requires progression mechanics to be real — without tracked relationship state, "slow burn" is just the user typing slowly.

OOC (out of character). Talking to the character-as-narrator rather than in-scene, usually bracketed: (OOC: skip ahead to the morning). Useful for steering without breaking the story.

Opening message / first scene. The creator-written scene that starts every story. On curated platforms it is authored and reviewed; its quality predicts the whole story's.

Lorebook / world rules. Structured world facts and constraints injected into scenes. Three sharp rules beat ten pages of lore — see creating characters.

Identity and media

Identity anchor. A fixed visual reference bound to a character so every generated image is the same person — the fix for visual drift, where prompt-based generation produces a different face each time. Explained fully in consistent character images.

Voice identity. The same principle for audio: one fixed voice per character across every call, rather than whatever the synthesizer produces today.

The business terms

Freemium / metered pricing. The two honest cost questions: is conversation itself free and un-degraded, and are prices visible before you spend? Lusto meters extras in Sparks, its transparent in-app currency. The four "free" models are dissected in what free actually gets you.

Versioned publication. Characters frozen as immutable versions at publish time, so a creator update cannot silently change a character mid-story. The quiet feature that prevents the category's worst betrayals.

Parasocial relationship. The old psychological term for one-directional bonds with media figures and fictional characters — the machinery AI companionship runs on, discussed honestly in can you fall in love with an AI.

MCP (Model Context Protocol). The open standard letting AI assistants use external tools — including, via lusto-mcp, searching and reading Lusto's characters and worlds from inside an assistant.