This is the formal term for what happens when you repeat a word until it loses its meaning.
Its opposite is semantic generation, which is what happens when you repeat a nonsense word until it seems meaningful.
Experimental psychologists study these effects. In fact, there is an article on this phenomenon in the American Journal of Psychology from 1907!
The rate of repetition affects the speed of meaning loss. This is explained as cortical fatigue, an actual biochemical effect, or by a theory about how attention oscillates. If you repeat the word absentmindedly while thinking of something else (absentmindedness, or echoic dislocation) then the meaning loss takes longer to happen.
Different words, fairly consistently, lose their meanings more quickly than others.
Different people also exhibit different tolerance levels for semantic satiation, not just on the individual level, but say, for children at different grade levels. Retarded children and very young children show a greater tolerance for word repetition than children of higher intelligence. This could have a very practical application in education.
This theory explains why I am ready to scream with boredom as I read Old Hat New Hat for the nine hundredth time to my fascinated year-old baby.
Semantic satiation is used in a somewhat horrible-sounding way, to treat people with Tourette's syndrome. People with Tourette's syndrome often yell out or repeat curses, uncontrollably.
The person with Tourette's is told to repeat their favorite obscenity as fast as possible. If they don't say it at least twice per second, they get a mild electric shock.
In Terry Carr's story "Stanley Toothbrush", the main character repeats the word "shelf" until it loses its meaning, and suddenly all the shelves in his house disappear. He has a semantic satiation psychic power! He also has the power of semantic generation, which becomes unfortunate for him when he and his girlfriend joke around about her imaginary lover, Stanley Toothbrush.
As a final note, semantic satiation explains why I never hear my mom telling me to clean my room.