A warm-up act, opening act, or supporting act is an entertainment act (musical, comedic, or otherwise), that performs at a concert before the featured act, or "headliner". Rarely, an opening act may perform again at the end of the event, or perform with the featured act after both have had a set to themselves.
The opening act's performance serves to "warm up" the audience, making it appropriately excited and enthusiastic for the headliner.
In rock music, the opening act will usually be an up-and-coming group with a smaller following than the featured artist. On long concert tours, different opening acts may be used for different legs of the tour.
The undercard is a similar concept in sports.