x1 is the similarity relation between all members of set x2 (set of quoted words) such that they all mean esentially one common thing x3 and are considered to be identical in any meaningful way in language x4.
Members of x2 need to be formally different words (could be expressed in IPA or careful pronunciation); some of them may not be, as transcribed, proper words in the language. If a language lacks a minimal pair which distinguishes paired phonés, then any strings which nominally differ in only such pairs will belong to x2 of this word in that language; in other words allophonic but not phonemic differences produce such similarities. But even with minimal pairs, some words may not be distinguishable. This word refers to indistinguishability in the form of a word as well as its meaning/usage; so, homophones are not necessarily similar in this sense, nor are synonyms. The combination of both qualities does produce such similarities in spoken English (not necessarily written English, since the orthography is not phonemic). In English, phonation duration and, in some cases, certain phonation qualities (such as tonation, voicing, nasalation, place of articulation, aspiration, etc. (again: only in some cases)) do not distinguish words. Dialectal differences usually produce such similarities too (a U.S. English speaker and a UK English speaker will say the word "car" differently but will agree about everything concerning it, including its rhyming qualities in their dialects, its meaning, and its spelling). In Lojban, phonation duration (such as ".yyy " versus merely ".y "; but notice that some transcriptions are interpreted as producing multiphthongs/glides/syllabification, so ".iii " is distinct from mere ".i ") and minor differences in place of articulation (trilled "r" versus English "r", for example) and sometimes voicing (for nasals, rhotics, laterals, and presently xy) do not produce word distinctions; so, they will generate such similarities in Lojban. Additionally any pair of words which come into gimkamsmikezypro (or similar for zi'evla) in Lojban have such a similarity in Lojban.