The sexual response cycle is a cycle that occurs in all human beings if they become sexually aroused and then go on to do things that stimulate them in a sexual way.  Scientists have identified four distinct phases to the human sexual response cycle.

The first phase of the sexual response cycle is known as excitement.  This phase can occur as a result of or as a precursor to actual sexual activity.  That is, a person can become excited through thoughts or sights or sounds without actually engaging in sexual activity.  In this phase, various changes in the body occur as the person becomes sexually excited.  These changes, as well as those in other phases of the cycle, can be seen at this link.

The second phase of the cycle is called plateau.  This stage goes from the time that one is excited until one is just about to have an orgasm.  This phase is typically achieved through actual sexual stimulation such as masturbation or various activities with a partner.

The third phase of the cycle is called orgasm.  This is when the person achieves the peak of sexual pleasure.  Unlike the other phases, which can last a long time, the orgasm phase typically only lasts a few seconds.

Finally, there is the phase that is called resolution.  In this phase, all of the changes in the body that have occurred during the rest of the cycle disappear and the body returns to its regular, non-excited state.

The sexual response cycle, then, is this sequence of events that occurs when a person becomes sexually aroused and then engages in some sort of sexual activity.