Antonyms of adv informally

1 of 2 senses of informally

Sense 1
informally -- (without formality; "he visited us informally")
       Antonym of formally (Sense 2)
      => formally -- (in a formal manner; "he was dressed rather formally")

Synonyms of adv informally

2 senses of informally

Sense 1
informally -- (without formality; "he visited us informally")

Sense 2
colloquially, conversationally, informally -- (with the use of colloquial expressions; "this building is colloquially referred to as The Barn")