According to the Clinical Psychology Review, there are at least 26 definitions of orgasm. In clinical terms, an orgasm is a sudden discharge of sexual tension that results in muscular contractions and changes in heart rate, blood pressure and breathing patterns.
In other words, an orgasm happens suddenly after building sexual tension, creating an intense sensation of pleasure. In men, orgasm may be accompanied by ejaculation.
There are lots of slang terms for orgasms. Cum or cumming is one of the most common.