# Why are regular languages called “regular”?

Why are regular languages (and from that regular expressions) called "regular"? There is lot of regularity also in context-free languages other types of languages.

I suppose that, in the beginning, the adjective "regular" has been used to differentiate that type of languages from other "non-regular" or somehow abnormal languages. If so, what where these other types, and what was their non-regularity?

• The Kleene's paper (an awesome paper, thank you!) contrasts "regular" events with "indefinite" events (that depend on another event that may have happened in an infinite past) and "definite" events (that depend on at most $p-l-1$ past moments). "Regular" events are those who depend on "the value" of the last moment $p$. – gioele Oct 26 '11 at 17:06