In musical theory an inversion is to write one or more notes of a chord an octave above its natural position.
For example, the root position of a triad of C major is:
http://meta.wikipedia.com/upload/c_triad.png
The first inversion means to shift the bottom note (the C) an octave higher:
http://meta.wikipedia.com/upload/inv2.png
The second inversion is to write the E an octave above:
http://meta.wikipedia.com/upload/inv3.png
The third inversion of a triad cannot be constructed, since shifting the G an octave higher would simply form the original root triad up an octave:
http://meta.wikipedia.com/upload/inv4.png
It is, however, possible to find the third inversion of chords which have four notes or more, such as the dominant seventh.
An Inversion is a meteorological phenomenon where (?cold air gets trapped above warm?), preventing air circulation. When inversions occur, air quality suffers.