Is there any way to perform the equivalent of a VLOOKUP so that a simple
mouseover (or click) of a cell displays the data in a pop-up like format?

The mainframe data at my work abbreviates everything, so unless you can
remember thousands and thousands of definitions, you have to provide them in
the spreadsheet somehow. Right now I have 12-15 columns of data. If I added
new columns alongside what I already have for the purpose of definitions,
that's at least 25 columns total, which I cannot possibly view on one (or
even two or three) screens at one time.

It would be great to instantly see the data by a mouseover, click, keyboard
stroke sequence, or something similar.

Any ideas?