ortho

adjective

or·tho ˈȯr-(ˌ)thō

: orthochromatic

Word History

First Known Use

1904, in the meaning defined above

Time Traveler

The first known use of ortho was in 1904

See more words from the same year

Cite this Entry

Style

“Ortho.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/ortho. Accessed 17 Apr. 2025.

Copy Citation

Medical Definition

ortho

1 of 2 adjective

or·tho ˈȯr-(ˌ)thō

1

: derived from or being an acid in the highest hydrated or hydroxylated form known

2

: relating to, characterized by, or being two neighboring positions in the benzene ring

ortho

2 of 2 adjective

: orthochromatic

More from Merriam-Webster

Play Quordle: Guess all four words in a limited number of tries. Each of your guesses must be a real 5-letter word.

Can you solve 4 words at once?

Can you solve 4 words at once?

Word of the Day

uncouth

See Definitions and Examples »

Get Word of the Day daily email!

Popular in Grammar & Usage

The Difference Between 'i.e.' and 'e.g.'

'Affect' vs. 'Effect'

Words You Always Have to Look Up

Democracy or Republic: What's the difference?

Every Letter Is Silent, Sometimes: A-Z List of Examples

Popular in Wordplay

What do SCOTUS, POTUS, and FLOTUS mean?

8 Words with Fascinating Histories

8 Words for Lesser-Known Musical Instruments

Birds Say the Darndest Things

10 Words from Taylor Swift Songs (Merriam's Version)

Popular

The Difference Between 'i.e.' and 'e.g.'

What do SCOTUS, POTUS, and FLOTUS mean?

8 Words with Fascinating Histories