An erogenous zone is an area of the body that excites sexual arousal when touched or stimulated. Primary erogenous zones are found in the mucocutaneous regions of the skin. Which other parts of the skin are sensitive to sexual touch is a very individual experience that depends also on factors like the kind of stimulation, the person stimulating, the right environment and the right time.
The mucocutaneous regions
The mucocutaneous (pronounced myū'kō-kyū-tā'nē-əs) zones are regions of the body comprising both mucosa and cutaneous skin. These mostly occur near the orifices of the body at which the external skin stops and the mucosa that covers the inside of the body starts. Such specific sites are externally visible sex organs (men: penis and scrotum; women: clitoris and vulva) and anus, lip, nipple and conjunctiva (a clear membrane that covers the white part of the eye and lines the inside of the eyelids).
Other erogenous zones
Examples are the (inner) thighs, buttocks, the sides and back of the neck, the axillas and the sides of the thorax. The pleasurable sensation perceived from these regions is simply an exaggerated form of tickle. The pleasant associations and the learned and anticipated responses concurrent with the stimulus produce the final amplified central sensation.