Definition of principal section
1 : a plane passing through the optical axis of a crystal; specifically : the principal plane that contains either the wave normal or the ray of light under discussion
2 : a plane perpendicular to the edge of an optical prism
Love words? You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.
Start your free trial today and get unlimited access to America's largest dictionary, with:
- More than 250,000 words that aren't in our free dictionary
- Expanded definitions, etymologies, and usage notes
- Advanced search features
- Ad free!
Seen and Heard
What made you want to look up principal section? Please tell us where you read or heard it (including the quote, if possible).