Erectile dysfunction is the inability to get and maintain an erection long and strong enough to complete sex in a way that is satisfying both the man and his partner.
Most men experience episodes when they cannot get an erection when they are very tired or stressed, and this is no cause for concern.
As they grow older however many men regularly experience difficulties in getting an erection, and this is the condition known as erectile dysfunction or ED.
About 75 per cent of ED is from physical causes and can be successfully treated in many cases, so it is a good idea to seek help. The longer it is left untreated the more likely it will become permanent and more difficult to reverse.
See a more detailed answer see What is erectile dysfunction?