Flaccid refers to the quality of being soft, or not firm or stiff. It is usually used in reference to a penis. A penis at rest, or not in a state of arousal, is referred to as being flaccid.
More About Flaccid
There are many causes of flaccidity in penises. Typically, a flaccid penis is the result of a lack of blood supply to the penis. In some cases, a penis may become flaccid during sexual activity. This can be caused by a variety of factors both psychological and physiological.