1.
to stop breathing for a short time :
Hold your breath and count to ten.
2.
to be anxious while you are waiting for sth that you are worried about :
He held his breath while the results were read out.
3.
( informal ) people say don't hold your breath! to emphasize that sth will take a long time or may not happen :
She said she'd do it this week, but don't hold your breath!