1 of 2


del·​toid ˈdel-ˌtȯid How to pronounce deltoid (audio)
: a large triangular muscle that covers the shoulder joint and serves to raise the arm laterally


2 of 2


: having a triangular shape
a deltoid leaf
see leaf illustration
: relating to, associated with, or supplying the deltoid

Did you know?

The fourth letter of the Greek alphabet is delta, and a capital delta is triangle-shaped. In English, delta commonly means the sand deposits that form a huge triangle at the mouth of certain large rivers. Deltoid as an adjective means "having a triangular shape", and botanists often use the word to describe the shape of certain leaves. The triangular, swept-back wings seen on jet fighter aircraft are called delta wings. Your deltoid muscles—not far from your rhomboids—form a cap on your shoulders, and some gym trainers even treat shoulder and deltoid as synonyms. Can you guess the general shape of deltoids when seen from the side?

Example Sentences

Recent Examples on the Web
The deltoids sit over the shoulder joint and connect to the humerus (upper arm) and the clavicle (collarbone), says Kalsched. Andi Breitowich, Women's Health, 18 May 2023 But my go-to arm routine was missing a major muscle group that (unfortunately) often goes unnoticed: rear deltoids. Andi Breitowich, Women's Health, 8 Mar. 2023 The blobs are labeled: Brain, deltoid, lung. Heather Lanier, Longreads, 10 Jan. 2023 Photos featuring the humble deltoid—that meaty muscle that swaddles the upper arm, newly famous as the injection site for all three currently cleared COVID-19 vaccines—have been flooding Twitter, Facebook, Instagram, and even Tinder. Katherine J. Wu, The Atlantic, 11 Mar. 2021 The anterior deltoid makes up the front head of the shoulder, and rotates the shoulder joint by drawing the arm inward. Tyler Hatfield, Men's Health, 26 Apr. 2022 Therefore, those receiving a vaccine in the thigh may need to use a longer needle (commonly 1.5 inch, as opposed to a 1 inch needle in the deltoid). Lois K. Solomon,, 6 Dec. 2021
Chest press targets the pectoral, deltoid, and tricep muscles. Alyssa Hui, Verywell Health, 28 Apr. 2023 The deltoid muscle has three parts: the anterior, the medial/lateral, and the posterior. David Otey, Men's Health, 10 Feb. 2023 Doctors are familiar with SIRVA from flu shots administered too high above the deltoid muscle. Lois K. Solomon, Sun Sentinel, 2 Nov. 2022 But a hardware malfunction from the initial surgery caused the suture that was holding his deltoid ligament to his medial malleolus to cause a stress fracture, which knocked him out of a second straight season. oregonlive, 27 Oct. 2022 At the center of it all is a deltoid-shaped bridge that contributes to the overall sense of harmony and symmetry of the design. Cait Bazemore, Robb Report, 19 Oct. 2022 In other words, these weight cutoffs are the best indicators of vaccine efficacy at different needle lengths, regardless of how much fatty tissue a person has around their deltoid muscles. Sarah Stark, SELF, 6 June 2022 During the 2½-hour process, originally expected to take 90 minutes, Anderson took out two screws that shredded the posterior tib tendon in McCormick’s ankle and put in tight rope to secure the deltoid ligaments. oregonlive, 10 Sep. 2021 Women also have thicker fat pads in the deltoid muscle—the part of the upper shoulder where the vaccine is injected. Emily Mullin, Smithsonian Magazine, 20 Apr. 2021 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'deltoid.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History



New Latin deltoides, from Greek deltoeidēs shaped like a delta, from delta

First Known Use


circa 1681, in the meaning defined above


circa 1753, in the meaning defined at sense 1

Time Traveler
The first known use of deltoid was circa 1681

Dictionary Entries Near deltoid

Cite this Entry

“Deltoid.” Dictionary, Merriam-Webster, Accessed 4 Jun. 2023.

Medical Definition


1 of 2 noun
del·​toid ˈdel-ˌtȯid How to pronounce deltoid (audio)
: a large triangular muscle that covers the shoulder joint, serves to raise the arm laterally, arises from the upper anterior part of the outer third of the clavicle and from the acromion and spine of the scapula, and is inserted into the outer side of the middle of the shaft of the humerus

called also deltoid muscle

see deltoid tuberosity


2 of 2 adjective
: relating to, associated with, or supplying the deltoid
the deltoid branch of the thoracoacromial artery

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