Emasculation is a term used to describe the reduction of a man's feeling of authority, power, or masculinity.

Men can feel emasculated by numerous means. Depending on the kind of man, different things can make one feel emasculated. Examples include;

Being spoken down to by another alpha male.
Your girlfriend making you look like pathetic, i.e. opening your ketchup bottle for you.
Your mother speaking to you like you're six in front of your friends.
Not being able to get a boner.
Reading Conservapedia and realising how some men are so overbearing about their masculinity it makes you feel pathetic for belonging to the same gender.
Expectations and pressure from friends, family and society in general.
Reduction in personal freedoms and liberties.
Failure to cope under pressure.