adj. 25B6; adjective
an informal discussion : UNOFFICIAL, casual, relaxed, easy-going, unceremonious; open, friendly, intimate; simple, unpretentious, easy, homely, cosy; informal unstuffy, laid-back, chummy, pally, matey.
an informal speech style : COLLOQUIAL, vernacular, idiomatic, demotic, popular; familiar, everyday, unofficial; simple, natural, unpretentious; informal slangy, chatty, folksy.
informal clothes : CASUAL, relaxed, comfortable, everyday, sloppy, leisure; informal comfy.
formal, official, literary, smart.