Two things are similar when they are close without being the same — your handwriting and your mother's, two shirts a shade apart on the rail, brothers you could mix up from across a street. Near, not identical.
The pattern to lock in is similar TO: this one is similar to that one. Not 'similar with', not 'similar as'. It also stands on its own — we had similar ideas, we come from similar backgrounds. The noun is similarity and the adverb is similarly, which opens a sentence adding a matching point: 'Similarly, the second test failed.' Hold the line between same and similar, because same means there is no difference at all and similar admits there is one.
1adjectivesimilarAlike in many ways but not exactly the same.
“Her new flat is similar to the old one, just brighter.”
synonyms: alike, comparable
2adjectivegeometryIn geometry, having the same shape but not the same size.
“The two triangles are similar, so their angles match exactly.”
Too few real examples of this word to estimate which sense is commonest.
Also accepted in games: alike
Similar came through French similaire and Medieval Latin similaris from Latin similis, 'like', a relative of simul, 'together' — the same family that stands behind simultaneous. Further back is a Proto-Indo-European root *sem-, the idea of oneness and sameness.
Proto-Indo-European
*sem-
Latin
similis — like
Medieval Latin
similaris
French
similaire
English, today
similar
From French similaire, from Medieval Latin similaris, extended from Latin similis (“like”); akin to simul (“together”).
Etymology from Wiktionary, CC BY-SA