Etymology is the origin of a word and the study of how its use and meaning have changed over time. For instance, some words with a Latin origin have variations to their plurals. Instead of adding -s, the following four words change form. Some of these uses have faded out almost completely during this century, so the more current English plural is shown in parentheses.
cactus, cacti (cactuses)
millennium, millennia (millenniums)
formula, formulae (formulas)
index, indices (indexes)
Another example through plurals is this handful of nouns that came to English through Greek: analysis, analyses; crisis, crises; neurosis, neuroses.
Online etymology pages and thorough dictionaries will explain what part of speech a word is, they will show its earliest appearance and original language, and they may list various places in literature where the word is found. If usage of the word has changed since its first appearance, this will also be noted. Below is an example using the word “parenthesis.”
From para- (“beside”), en- (“in”) and tithenai (“put” or “place”). The earliest mention of this word was in the 1540s, the Middle French “parenthèse,” which refers to a word or clause inserted into a sentence. In 1715, curved brackets were used to set aside the inserted words and clauses. “The Dictionary of Buckish Slang,” London, 1811, shows this word was once used as slang to refer to the pillory (wooden) or a prison (iron).