A term is a name, expression or word used to describe some particular thing. It is also a period of time, such as in the case of a loan. It also describes the specifics of an agreement, such as the terms of the lease.
There is no such word as termology so I assume you mean terminology. Terminology = The vocabulary of technical terms used in a particular field, subject, science, or art; nomenclature eg Terminology in Computing : bit, byte, cpu, dongle, mainframe, bug, LAN, compiler, etc
the length of time a certain position is held. example: obama will only serve one term
A word that serves as a key, as to the meaning of another word, sentence, passage, etc.
because it is
THERE IS NO key term