Why is the ninth month called September?
1. One of a fixed set of symbols built into the syntax of a language. Typical keywords would be if, then, else, print, goto, while, switch. There are usually restrictions about reusing keywords as names for user-defined objects such as variables or procedures. Languages vary as to what is provided as a keyword and what is a library routine, for example some languages provide keywords for input/output operations whereas in others these are library routines.
2. A small set of words designed to convey the subject of a technical article. Some publications specify a fixed set of keywords from which those for a particular article should be chosen.