What is a tuple?

My apologies for the stupid question, but before I started using
postgres I never came across the word "tuple" quite in this context
before. I know  a "tuple" as "a data object containing two or more
components" ... but I'm not sure of its precise meaning in the
postgres universe. Is a tuple a row, a field value, a field value
paired with its datatype, what? If someone asks me the size of my
largest tuple, how do I calculate it? It seems to be related deeply
to the structure of postgres somehow.

Thanks for your patience.

Elaine