The player with the longest trump holding has trump control. When two players have equally long trump suits, the player whose partnership has the lead has the tempo and therefore has trump control. On deals where one of the defenders has four trumps, there can be an intriguing battle for trump control.
South plays in 4♠. West leads the ♦5. East wins with the ♦A and returns the ♦3.
Suppose that South ruffs and plays a trump. West then wins and plays a third diamond, forcing South to ruff again. Declarer has trump control because West and North have the same number of trumps and declarer has the lead.
Now suppose the play starts the same way but West holds up the ♠A until the third round before continuing diamonds. This time West has the lead while North and West have equal trump length, so West has trump control.
Finally suppose that declarer (correctly) discards a heart at trick two. This way West is unable to set up a force (the ♦Q will be high) and declarer has trump control.