Zero Anaphora

Zero anaphora is the use of a gap, in a phrase or clause, that has an anaphoric function similar to a pro-form. 
It is often described as “referring back” to an expression that supplies the information necessary for interpreting the gap. Examples (English) 

• There are two roads to eternity , a straight and narrow, and a broad and crooked.

In this sentence, the gaps in a straight and narrow [gap], and a broad and crooked [gap] have a zero anaphoric relationship to two roads to eternity.