Extremely foolish term in computing (a field well-known for inventing idiotic jargon instead of using perfectly good terminology that's been in use often for more than 100 years).

A table (in this technical sense) is a geometric arrangement of data, usually and in this case orthogonally in two dimensions:

   key   |  entry1  | entry2  ...
         |          |
         |          |
         |          |
Given a key value, you may find a row with that key and read off the entry or entries. This is called looking it up.

Anything else you can do with a table? No, not really. If you really want to get picky, you can also create a new row with a key value. Actually, you probably have to do that in any useful table that's not automatically initialised to the right size.

So why is a table where you look things up known as a lookup table in various parts of computing?

It even has its own abbreviation: LUT. Furrfu!