The definition of erectile dysfunction (ED). Erectile dysfunction (ED) is the inability to achieve and sustain a firm enough erection for sexual activity. Approximately one in ten men will develop ED at some time in their lives. The fact that ED is typically a sign of another, underlying disease must always be kept in mind. A lack of desire, problems with orgasm and ejaculation, and ED are all conditions that are not considered normal at any age and can coexist with one another to impair sexual activity.