Any arbitrary unit of time as used in all television iterations of the Transformers. An Astrosecond can range from one Earth second to 1 billionth of a second, as defined by the writers of that episode.
The astrosecond was first introduced in Season 1 Episode 4: Transport to Oblivion, when Shockwave states that his Space Bridge will be inoperable for a period of 3,000 astroseconds.
In the very next episode, Roll For It, Megatron tells Starscream to meet him at Chip Chase's Lab in five billion astroseconds.
Even with the generous assumption that it took screen time (2.5 minutes) for the Autobots to drive halfway across the continental United States and be attacked by the Decepticons at the cave 79 miles from the highway, the five billion astroseconds it would take for Starscream to find replacement parts is equivalent to 389 years of Earth time if we use the definition from Transport to Oblivion. It takes place in less than 7 minutes of screen time.
Episodes in which Astroseconds are used:
Note that this is an incomplete list and as I continue to watch the series I'll add more titles.
Transport to Oblivion
Roll For It
The Ultimate Doom: Part 2
I'm surprised there arent more references to the Astrosecond on the Net.
Pages I found useful:
sekampa.tripod.com/hoist/hr-rollforit.html
cybertronchronicle.freewebspace.com/episodeguide/ ultimatedoom2-script.html