Trace

A trace marks the location in a sentence or syntactic tree from which a lexical item has been moved.

Examples
From the original structure, wh-movement creates a trace where John used to be.
 * John will eat apples.
 * [Johni [will eat apples]]
 * Who will eat apples?
 * [Whoi [ti will eat apples]]

CP 	      /| Whoi | IP               /| NP	I' |	|\ 	    |	I VP 	     ti	|  | | V' 	      will |\ V \ | NP                    eat | | 		    apples