principal section

noun

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!