The word webcast is derived from "web" and "broadcast". Its use has varied since the early-mid 1990s as the nature of the medium came into public use.
(Note: There are possibly two key definitions, one in National Association of Broadcaster's publications/training manuals that says webcasting is sending audio and/or video live over the Internet or on digital networks, as well as listening or watching on-demand files. (Link Needed to actual definition) The FCC also has a definition of webcast. (Link Needed to actual legal definition.)
The generally accepted use of the term webcast is the "transmission of linear audio or video content over the Internet".