circa

Circa takes the ACCUSATIVE, always, and never anything else. Its plain sense is spatial, "round about, in the neighbourhood of", and from there it slides two ways: with numbers, dates and times it softens to "roughly, approximately", and with a topic it means "concerning", circa hanc rem.

Latin also has circum, likewise with the accusative and stricter about actually encircling something, and the adverb circiter for rough numbers. Circa gradually took over both jobs, which is why it is the form that survived into scholarly English as the "circa" or "c." in front of a date.

It doubles as an adverb, with no noun after it, meaning simply "round about". The family is large and useful: circus, circulus, circuitus, and the prefix circum- in circumvenio, circumspicio and every English circum- word.