Short answers, pulled from the story.
An original net animation (ONA) is an anime that is released directly onto the Internet rather than through television broadcast or physical media. ONAs may also later air on television as long as the initial release was online. The term mirrors "original video animation," which the anime industry has used since the early 1980s for straight-to-video releases.
Makoto Shinkai pioneered ONA in the late 1990s by producing animated short films on a home computer and distributing them on the Internet. His earliest works include Toi Sekai (1997), Kakomareta Sekai (1998), and She and Her Cat (1999).
Early ONA web series include Infinite Ryvius: Illusion (2000), Ajimu (2001), and Mahou Yuugi (2001). These appeared as broadband Internet speeds began to make online video distribution practical in Japan.
ONAs tend to be shorter than traditional anime episodes, sometimes running only a few minutes. Hetalia: Axis Powers is a well-known example of an ONA with very short episodes. This began to change in the second half of the 2010s as full-length series were licensed exclusively for streaming platforms.
Netflix, Amazon Prime Video, and Disney+ began licensing full anime series exclusively for their platforms in the second half of the 2010s, bringing the ONA format to a global audience and shifting it away from its short-episode origins.
An ONA (original net animation) is anime released directly on the Internet, while an OVA (original video animation) is anime released on physical video formats. The ONA term was intentionally modeled on the OVA label, which has been in use in the anime industry since the early 1980s.