TPAC historically stands for Technical Plenary and Advisory Committee, since in its initial physical format, it gathered in a single place a meeting of the W3C Advisory Committee (where each W3C Member has a representative) and a broad set of technical group meetings, culminating in a plenary day – the technical plenary.

In recent years, the plenary format had mostly been replaced by an unconference format organized as a series of one-hour sessions on topics brought by the attendees. In its current virtual format, there isn't as much of an opportunity for a ‘plenary’ meeting in any case, and for practical reasons, the Advisory Committee meeting part of the event is held somewhat apart from the rest of the technical meetings.

The name TPAC is familiar enough to regular W3C participants that we are still using the name in these new circumstances, but we are trying to reference it more broadly as ‘W3C Annual Conference’ to make it easier to understand.