Not happening at the same time as something else.
Nonsynchronous things don't match up in time.
Examples:
The two clocks are nonsynchronous; one is fast and the other is slow.
Her clapping was nonsynchronous with the music.
The nonsynchronous arrival of the buses caused confusion.
The nonsynchronous dancing made everyone laugh.
In the nonsynchronous race, no one knew who won.
The nonsynchronous fireworks were beautiful.