What is a relation in database terminology?

A relation is a set of unique tuples, where a tuple consists of an entity ID value which is RELATED TO (identifies) one or more attributes. It is NOT a table, which is a different level entirely (implementation rather than design).

I can't make this definition any shorter without leaving something out, but it is so short as to be merely a list of terms. If I make it longer, I will probably confuse the main point which is: "what does the word relation mean in this context? What is being related?"