As a non-teacher, I would say that, in the end, usage dictates meaning. Humpty said, "When _I_ use a word, it means just what I choose it to mean -- neither more nor less.'" and this really sums up the relationship between usage and meaning. Here are some examples:
Decimate has as a meaning, "kill one out of every ten". Some people insists that this is still the ONLY acceptable usage of the word. However, widespread and long-term usage has seen the word take on a broader meaning. In fact, some even say that the meaning has nearly reversed, from "kill 1 out of every 10" to effectively meaning "kill 9 out of every 10". The now somewhat overworn and tired English word "nice" once meant "ignorant" or "foolish". Its meaning changed as its usage changed.
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