What do you call it when you add zeros between the elements of a vector?
In the context of filter banks, simply upsampling (without filtering, or with the trivial all-pass filter, as Wikipedia relates it to interpolation), denoted by an up-arrow, as illustrated below fro a 2-fold or order-2 upsampling:

 

This is described in Filter banks: Decimation, Interpolation, and Modulation, and the  interpolation filter associated with upsampling is called synthesis filter. 

As noted by @Jason R, one also finds "expansion". Expansion is also found as a streching for continuous signals, while compression is the effect in the dual Fourier domain. Sometimes, one can find "zero-insertion" or "zero-stuffing".

Actually in image processing some use "upsampling" as a proxy for pixel increase with filtering.