From The Art and Popular Culture Encyclopedia
Nervous laughter is laughter evoked from an audience's expression of embarrassment, alarm, or confusion, rather than amusement. Nervous laughter is usually less robust in expression than "a good belly laugh", and may be combined with confused glances or awkward silence on the part of others in the audience. Nervous laughter is considered analogous to a courtesy laugh, which may be rendered by more of a conscious effort in an attempt to move a situation along more quickly, especially when the comedian is pausing for laughter.
Unhealthy or "nervous" laughter comes from the throat. This nervous laughter is not true laughter, but an expression of tension and anxiety. Instead of relaxing a person, nervous laughter tightens them up even further.