A formal Language is recursively defined set of strings on a fixed alphabet. Some parts of it can correspond to natural languages like English.
Terms
Linguistic items whose function is to denote a person or object are known as terms, represented as lower-case letters with or without numerical subscripts. Terms usually used letters from the beginning of the alphabet such as a, b, c ect.
Variables
Again, represented by lower-case letters, near the end of the alphabet, with or without numerical subscripts. E.g. x, y, z etc.