Head movement

Head Movement is a movement of a head, an X0, out of its projection to another head-position.

Example
in questions a finite verb may move to C0. In (i) the auxiliary will (an I, see INFL) is moved to C0, leaving a trace in I.

(i) [CP What [C willi] [IP John [I ti] buy]]?

Head movement obeys the Head movement constraint.

Link
Utrecht Lexicon of Linguistics