Simple Definition of bacillus

from our Learner's Dictionary

bacillus

noun

bəˈsɪləs

plural bacilli -ˈsɪˌlaɪ

technical : a straight rod-shaped bacterium that requires oxygen for growth

also : a bacterium that causes disease

Cite this Entry

Style

“Bacillus.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/bacillus. Accessed 20 Jun. 2026.

Copy Citation

More from Merriam-Webster

Word of the Day

longueur

Listen to the pronunciation of longueur

See Definitions and Examples »

Get Word of the Day daily email!

Popular in Grammar & Usage

'Canceled' or 'cancelled'?

5 Verbal Slip Ups and Language Mistakes

Is that lie 'bald-faced' or 'bold-faced'?

'A' or 'an'? What about before 'h'? An Indefinite Article Guide

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

Popular in Wordplay

One-Word Oxymorons: Bittersweet, Spendthrift, and More

8 Obscure Words for Sleepy Times

Cattywampus, Taradiddle, and More Funny-Sounding Words

The Words of the Week - Jun 19

12 Lovely and Unusual Words for the Natural World

Popular

'Canceled' or 'cancelled'?

One-Word Oxymorons: Bittersweet, Spendthrift, and More

8 Obscure Words for Sleepy Times