Elementary means of the simplest and most basic kind. The first things, before anything is built on top.
It describes knowledge and mistakes alike, so an elementary error is one nobody should make and elementary physics is the beginning of the subject. American schools use it for the first stage, so elementary school covers roughly ages five to eleven. Sherlock Holmes never actually said the famous phrase in the books, though he did use the word. Its relatives are element and elemental, and elemental means primal rather than simple.
1adjectiveelementaryBasic and introductory.
“The course covers elementary statistics.”
2adjectivesimpleSo simple that it should be obvious.
“It was an elementary mistake.”
Too few real examples of this word to estimate which sense is commonest.
Elementary is about the elements. Latin elementum meant the four elements of antiquity and, by extension, the fundamentals of anything, so an elementary lesson teaches the earth, air, fire and water of its subject.
Latin
elementārius — elementary
Middle English
elementare
English, today
elementary
Built from
From Middle English elementare, from Latin elementārius (“elementary”), from elementum (“one of the four elements of antiquity; fundamentals”) + -ārius. Cognate with French élémentaire. by surface analysis, element + -ary.
Etymology from Wiktionary, CC BY-SA