An ONA (Original Net Animation) is an anime released first, and usually primarily, on the internet: a streaming platform or video site rather than television, cinema, or disc. It is the streaming-era counterpart to the OVA.
The ONA is the internet-native sibling of the OVA. Where the OVA belonged to the video store and the disc bundle, the ONA belongs to the platform premiere: a series released to a streaming service or video site as its first and primary home, on the platform's schedule rather than a broadcaster's.
That distribution changes the format's shape. ONAs are free of the cour grid, so episode counts and lengths vary: short-form series of a few minutes per episode, full series dropped all at once for binge release, and everything between. The cour vocabulary fits them awkwardly, and databases often list their runs by release batch instead.
For canon and continuity nothing changes: an ONA is a release route, not a status. A streaming-first series is exactly as canonical as its source and creators make it, and a streaming-first special episode is the modern descendant of the disc-bundled OVA. The label tells you where a work premiered, never what it is worth. The category also covers short experimental releases, web-only spin-offs, and platform-exclusive episodes, which means databases argue about ONAs the way they once argued about OVA boundaries.
In use"A series that premiered as a full-season drop on a streaming platform is an ONA release."