A sensation so intensely pleasurable that it is not unlike an orgasm.
Orgasmic sensations may be considered to be many different types of events including taste, smell, sound, sight, and types of touching both sexual and non-sexual in nature. Often for something to be considered orgasmic, in addition to the event being intesely pleasurable it should be coupled with a release of tension which can also be either sexual or non-sexual in nature. Orgasmic events are at considered to be at least midly erotic, even if only on a subconcious level.
To use the work orgasmic in a sentence:
"That gooey dark chocolate brownie-cake is orgasmic!"
Other things that may be considered orgasmic: perfumes (esp. Acqua di Gio), tropical flowers, hot springs, fireworks, etc.