Synonyms for Informal:

adj.

all (adjective)
loose.
casual, simple (adjective)
congenial, mixed, natural, unofficial, homey, intimate, relaxed, unrestrained, everyday, democratic, ordinary, colloquial, unconstrained, extempore, straightforward, motley, unconventional, folksy, cool, free-and-easy, unceremonious, loose, frank, spontaneous, easygoing.
colloquial (adjective)
conversational, idiomatic, slang, vernacular, colloquial, casual.
succinct (adjective)
ordinary, stark, unpoetical, intelligible, blunt, simple, honest, prosaic, Plain-spoken, candid, bald, uncluttered, folksy, unpretentious, unadorned, clear, frank, direct, outspoken, straightforward, casual.
talkative (adjective)
conversational, chatty, Confabulatory.

n.

natural, the more the merrier, spontaneous, triumphal, by invitation (only), easygoing, unrestrained, white tie, relaxed, laid-back, bacchanalian, tighten, black tie, BYOB, plain. words, at ease, composed, nonchalant, Confabulatory, calm, at one with, chatty, chilled, cool. tiered, regulatory, established, self-regulatory, structural, structured, flat, governing, central. informal (noun)
cozy, intimate, conversational, subliterary, unceremonial, loose, unliterary, unceremonious, common, vulgar, everyday, slangy, epistolatory, vernacular, nonliterary, epistolary, folksy, colloquial, unofficial, friendly, free-and-easy, casual.

Other synonyms:

Confabulatory, bacchanalian, structured, white tie, BYOB, laid-back, self-regulatory, regulatory, relaxed, triumphal. established, easygoing, structural, tiered, governing. central. spontaneous, unrestrained. natural. flat. cozy
cozy.
intimate
chatty, intimate.

Usage examples for informal

  1. I have always believed that some informal arrangement had been made between the parties concerned during the Major's late visit to England. – The Narrative of a Blockade-Runner by John Wilkinson
  2. These were altogether informal and gave but little satisfaction to the merely fashionable folks who now and then attended them. – Recollections and Impressions 1822-1890 by Octavius Brooks Frothingham