In syntax, a gap that is dependent on another gap. By gap is meant a place where a component of a sentence used to be, but which has been moved away. Ordinary gapping is illustrated by:
John read a book.
John read something.
What did John read ?
When you question something in English you put a question word at the front of the sentence. Where the answer would go in the statement is a gap. The verb 'read' needs to be followed by an object; and the question 'what' has to be understood as being the object of 'read' (not its subject, or any other aspect). So the question is analysed as leaving behind a trace at the original position, not pronounced but still grammatically active, co-indexed with the question-word:
Whati did John read ti?
Sentences often contain pronouns referring back to items earlier in the sentence:
John read the booki without buying iti.
A parasitic gap occurs when one of these already co-indexed elements is moved to the front to form a question:
What did John read without buying?
Whati did John read ti without buying ti?
There are now three co-indexed elements, all referring to the same thing, but two of them are traces. There is a chain of traces: the final one, which used to be 'it', is bound to the earlier trace position, and that's bound to the question word at the front. Note that normally both co-indexed items get gapped out. It's either ungrammatical or at least less acceptable/natural to keep the 'it' and say
*What did John read without buying it?
Another odd thing about parasitic gaps is that they can occur at long distances where, if there wasn't this long chain, ordinary questions can't be made. To see this confusing claim explained, look at these:
John lost the book before reading it.
John lost the watch before reading the book.
What did John lose before reading?.
*What did John lose the watch before reading?
The final question is ungrammatical. You can't extract the object 'the book' from after 'reading' and move it across the rest of the sentence to question it with 'what'. But you can move 'the book' from the higher position and gap out the 'it' after 'reading'.