1. A common spoken name for "!" (ASCII 33), especially when used in pronouncing a bang path in spoken hackish. In elder days this was considered a CMUish usage, with MIT and Stanford hackers preferring excl or shriek; but the spread of Unix has carried "bang" with it (especially via the term bang path) and it is now certainly the most common spoken name for "!". Note that it is used exclusively for non-emphatic written "!"; one would not say "Congratulations bang" (except possibly for humorous purposes), but if one wanted to specify the exact characters "foo!" one would speak "Eff oh oh bang".
2. An exclamation signifying roughly "I have achieved enlightenment!", or "The dynamite has cleared out my brain!" Often used to acknowledge that one has perpetrated a thinko immediately after one has been called on it.