|Formal||- having precise meaning or objective criteria of correctness, ideally computer verifiable, based simply upon "syntactic" form ;|
This is the sense of the word most relevant to our endeavor, and is perhaps the most common sense of "formal" used in the literature of logic and analytic philosophy.
This usage stands in contrast with other common meanings as being rigid, ceremonious, solemn, customary or not casual. It is closer to meaning exact, methodical or orderly, but for the purposes of supporting precise understanding or procedures that can be performed by machines.
By "informal" we simply mean not formal, in this sense.
As is explained in