A bid made at a high level due to the presence of interference bidding. For example after a 1♠ opening and a 3♣ overcall, responder may have to bid 3♠ with 7-8 points and some spade support, less than that required for an uncontested 3♠ bid. Such a bid is a ‘pressure bid’.
If holding full values for a 3♠ raise, responder would find another bid to take the pressure of opener, such as jumping to 4♠.
The usual rule is to allow the opposing bidding to pressure you into bidding one level higher than you would have bid without their bidding.
You would have bid only 2♠ if South had passed, so you should bid 3♠ over 3♥ but not 4♠ over 4♥.