matrix-spec/event-schemas/schema/m.room.topic

{
    "type": "object",
    "title": "Topic",
    "description": "A topic is a short message detailing what is currently being discussed in the room.  It can also be used as a way to display extra information about the room, which may not be suitable for the room name. The room topic can also be set when creating a room using ``/createRoom`` with the ``topic`` key.",
    "allOf": [{
        "$ref": "core-event-schema/state_event.json"
    }],
    "properties": {
        "content": {
            "type": "object",
            "properties": {
                "topic": {
                    "type": "string",
                    "description": "The topic text."
                }
            },
            "required": ["topic"]
        },
        "state_key": {
            "type": "string",
            "description": "A zero-length string.",
            "pattern": "^$"
        },
        "type": {
            "type": "string",
            "enum": ["m.room.topic"]
        }
    }
}