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• tertiary
  tertiary

  adjective

  of the third order, rank, stage, formation, etc.; third.
• Tertiary
  Tertiary

  adjective

  of, denoting, or formed in the first period of the Cenozoic era, which lasted for 63 million years, during which mammals became dominant

tertiary

American  

[tur-shee-er-ee, tur-shuh-ree] / ˈtɜr ʃiˌɛr i, ˈtɜr ʃə ri /

adjective

1. of the third order, rank, stage, formation, etc.; third.

2. Chemistry.

   1. noting or containing a carbon atom united to three other carbon atoms.

   2. formed by replacement of three atoms or groups.

3. (initial capital letter) noting or pertaining to the period forming the earlier part of the Cenozoic Era, occurring from 65 million to 2 million years ago, characterized by the development and proliferation of mammals.

4. Ornithology. tertial.

5. Ecclesiastical. noting or pertaining to a branch, or third order, of certain religious orders that consists of lay members living in community regular tertiaries or living in the world secular tertiaries.

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noun

tertiaries plural

1. (initial capital letter) the Tertiary Period or System.

2. Ornithology. a tertial feather.

3. (often initial capital letter) a member of a tertiary branch of a religious order.

4. tertiary color.

Tertiary ¹ British  

/ ˈtɜːʃərɪ /

adjective

1. of, denoting, or formed in the first period of the Cenozoic era, which lasted for 63 million years, during which mammals became dominant

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noun

1. the Tertiary period or rock system, divided into Palaeocene, Eocene, Oligocene, Miocene, and Pliocene epochs or series

tertiary ² British  

/ ˈtɜːʃərɪ /

adjective

1. third in degree, order, etc

2. (of education) taking place after secondary school, such as at university, college, etc

3. (of an industry) involving services as opposed to extraction or manufacture, such as transport, finance, etc Compare primary secondary

4. RC Church of or relating to a Third Order

5. chem

   1. (of an organic compound) having a functional group attached to a carbon atom that is attached to three other groups

   2. (of an amine) having three organic groups attached to a nitrogen atom

   3. (of a salt) derived from a tribasic acid by replacement of all its acidic hydrogen atoms with metal atoms or electropositive groups

6. Also called: tertial.  rare ornithol of, relating to, or designating any of the small flight feathers attached to the part of the humerus nearest to the body

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noun

1. Also called: tertial.  rare ornithol any of the tertiary feathers

2. RC Church a member of a Third Order

tertiary Scientific  

/ tûr′shē-ĕr′ē /

1. Tertiary. The first period of the Cenozoic Era, from about 65 to 2 million years ago. During this time the continents took on their present form, and the climate changed from being warmer and wetter, in the early part of the period, to being drier and cooler in the later part. Mammals replaced dinosaurs as the dominant form of terrestrial animal life, and many modern types of flowering plants, insects, mollusks, fish, amphibians, reptiles, and birds appeared. The Tertiary is subdivided into the Paleogene and the Neogene, although these terms are not as widely used as are the names of the epochs that constitute them.

2. See Chart at geologic time

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1. 1. Relating to or having a carbon atom that is attached to three other carbon atoms in a molecule.

   2. Relating to an organic molecule, such as an alcohol, in which the functional group is attached to a tertiary carbon.

2. Relating to an advanced level of medical care, usually provided by subspecialists after the delivery of primary medical care.

3. Compare primary secondary

Other Word Forms

Derived Forms

• post-Tertiary
  adjective
• pre-Tertiary
  adjective

Inflected Forms

noun

• tertiaries
  plural

Etymology

Origin of tertiary

1540–50; < Latin tertiārius of third part or rank, equivalent to terti ( us ) third + -ārius -ary

Explanation

Tertiary is another way of saying "third in importance," like socializing with co-workers being a tertiary reason for getting an after-school job — less important than, first, earning money and second, gaining skills. To correctly pronounce tertiary, say "TER-she-err-ee." If you are the third child born in your family, don't be tempted to call yourself the "tertiary child." This means you are less important that your two older siblings. However, in some cases, tertiary does not have to do with ranking third — the Tertiary period marks the beginning of life for mammals, and in the United Kingdom, tertiary education means "college-level."

Example Sentences

Investors instead have been gravitating to what Morgan Stanley analyst Joe Moore described as “secondary and tertiary AI beneficiaries” that are also benefiting from the booming investment cycle.

From The Wall Street Journal • May 21, 2026

"The drug creates an immune microenvironment within the tumor, and essentially replaces the tumor with these tertiary lymphoid structures."

From Science Daily • Mar. 16, 2026

That has secondary and tertiary effects, because spending on digital infrastructure boosts demand for power turbines and cooling equipment, as well.

From Barron's • Dec. 31, 2025

But if that’s the case, “Hamnet” should spend far more time fleshing out its titular character than its tertiary ones, letting the viewer see so much more of little Hamnet’s life than we’re ultimately afforded.

From Salon • Nov. 30, 2025

Originally, geological history was divided into four spans of time: primary, secondary, tertiary, and quaternary.

From "A Short History of Nearly Everything" by Bill Bryson