A breakout session is one of several smaller sessions running at the same time, letting attendees pick the topic or track that fits them.
One room cannot serve a mixed audience. Breakouts let a conference run beginner and advanced content at the same time, or serve three industries without boring two of them. The cost is that every attendee misses most of your program, which is why recordings and session notes matter more than organizers expect.
Attendance across concurrent breakouts is never even. One session will draw triple the others, and it is rarely the one you predicted. Pre-registration for sessions is the only reliable way to know, and even then expect drift on the day. Oversizing every room is expensive; undersizing the popular one is the thing attendees complain about.
Same format, different word. Breakout is the clearer default for a broad audience, and parallel session is worth recognizing when you are writing for a scientific or association program.
Sessions carry their own capacity limits, attendees can register for sessions during ticket purchase or later, sessions can be made private so only pre-registered attendees see them, and admins and speakers can still enter a session that has hit its cap. See event agenda builder.