In algebra, what is a term?

That depends on the context, but in general a term is a number or a letter (or a combination thereof) separated by + or -.

Examples of terms:

x, 2x, a, abc, 7

In case of something like (2x-1), this can be considered as one term as a part of a larger expression, as two terms as within the brackets, or even as three terms as distinct values. The latter is rare, though.