without formality or ceremony; casual

an informal visit.

not according to the prescribed, official, or customary way or manner; irregular; unofficial

informal proceedings.

suitable to or characteristic of casual and familiar, but educated, speech or writing.

Grammar . characterizing the second singular pronominal or verbal form, or its use, in certain languages

the informal tu in French.

adjective

buttoned-up , ceremonious , dressed-up , formal , official , rigid , stiff

adjective

breezy , colloquial , congenial , cool * , democratic , down home , easy , easygoing , everyday , extempore , familiar , folksy , frank , free , free-and-easy , homey , improv , inconspicuous , intimate , laid back , loose , low-pressure , mellow , mixed , motley , natural , off-the-cuff , open , ordinary , relaxed , spontaneous , sporty , straightforward , throwaway , unceremonious , unconstrained , unconventional , unfussy , unofficial , unrestrained , urbane , without ceremony , casual , chatty , confabulatory , en famille , irregular , simple , unorthodox