The event fired when a Video begins is "mediaBegin" (constant in Player API SWC is MediaEvent.BEGIN, constant in JavaScript API is BCMediaEvent.BEGIN),
This is dispatched when the media begins playing the first time, post buffer. This is the equivalent to the old "streamStart" and should be used for any tracking purposes. It will only be dispatched once for a media object, so not on replay (unless the media is loaded a second time after another piece of media is played in between).
A stream means the start of the delivery of a media file as initiated by an end user or by an auto play feature of a Brightcove player where the transfer of the media file is through streaming delivery, progressive download delivery or any other format supported by Brightcove. A stream can be all or a fraction of a single media file and there may be multiple Streams in an end user's session. If the same media file, or portion thereof, is re-viewed by and end user in a single session or in a multiple session, each view counts as a separate stream. Bumpers and ads do not count as streams.