Merriam-Webster Logo
SINCE 1828
Menu
dictionary thesaurus

principal section

noun

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!

Learn More about principal section

Statistics for principal section

Look-up Popularity

Comments on principal section

What made you want to look up principal section? Please tell us where you read or heard it (including the quote, if possible).

Show Comments  Hide Comments 

WORD OF THE DAY

spoonerism

Listen to the pronounciation of spoonerism

transposition of initial sounds of words

Get Word of the Day daily email!

Test Your Vocabulary

Word Puzzles

  • alt 57589b2b4c6a3
  • Which answer pairs best with each of the following words? living, estate, club, music, road, mile
Spell It

Can you spell these 10 commonly misspelled words?

TAKE THE QUIZ
Syn City

Test Your Knowledge - and learn some interesting things along the way.

TAKE THE QUIZ
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

Words at Play
Ask the Editors
Word Games