1. 1. Of or relating to the foundation or base; elementary: the fundamental laws of the universe.
   2. Forming or serving as an essential component of a system or structure; central: an example that was fundamental to the argument.
   3. Of great significance or entailing major change: a book that underwent fundamental revision.
2. Physics.
   1. Of or relating to the component of lowest frequency of a periodic wave or quantity.
   2. Of or relating to the lowest possible frequency of a vibrating element or system.
3. Music. Having the root in the bass: a fundamental chord.

1. Something that is an essential or necessary part of a system or object.
2. Music. The first harmonic in a harmonic series; the lowest harmonic.
3. Physics. The lowest frequency of a periodically varying quantity or of a vibrating system.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.