It generally refers to any item of a series of items that are added together (or subtracted - in Algebra, subtraction can be seen as a special case of addition). For example, in a + b, "a" is a term; so is "b". In x2 + 7x + 5, x2 is one term, 7x is another term, and 5 is a third term.
Photosynthesis is the term used to describe how green plants or bacteria use light energy to convert water and carbon dioxide into sugar. I believe it comes from two Greek words - for light and putting together or synthesising.