konato

Take it apart and the word explains itself. Koni is to know a person; -at- is the passive participle, "being ...ed"; -o makes a noun. A konato is therefore literally "a person who is known" - which is exactly what an acquaintance is, someone you know without the claim of friendship. English needs a Latin loanword for the idea; Esperanto assembles it from three pieces a beginner already has.

The tense of the participle is doing real work. Konato is someone known now, konatino an acquaintance who is a woman, and konatiĝi means to get to know one another, to become acquainted. Swap the participle and you change the timing rather than the word: this whole family of participle-plus--o nouns is set out in participial nouns, where the same machinery gives amato (a beloved), sendito (an envoy) and akuzito (a defendant).