What is meant by "Expression" of Non-Coding RNA?
With many non-coding RNAs, the RNA is the functional endpoint. Therefore, ncRNA "expression" simply refers to the production of that functional component. Similarly to with proteins, this involves looking at differential tissue production of that noncoding RNA (i.e. in which tissues the RNA is produced).

Gene expression is defined in the Oxford Dictionary of Biology as:


  The manifestation of the effects of a gene by the production of the
  particular protein, polypeptide, or type of RNA whose synthesis it
  controls.


In other words, gene expression means the same thing whether the functional product is a protein or a noncoding RNA. Of course, there is no translational component to expression of ncRNA.