Plural of Stimulus

What Is the Plural of Stimulus?

The plural of "stimulus" is "stimuli."

• In art, stimuli are things that interests artists and give them new ideas.
• Rapid changes, called stimuli, provoke reactions in living organisms that are often very different from those expected from non-living things. (Professor John Blamire)

The noun "stimulus" has a Latin root, which is the derivation of the plural "stimuli." "Stimuluses" (which adheres to the standard rules for forming plurals) is not an accepted plural.

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The Standard Rules for Forming the Plurals

The table below shows the standard rules for forming the plurals of nouns in English.

Type	Example of Type	Forming the Plural	Plural
Most Nouns	lamp scythe	add s	lamps scythes
Noun Ending s, sh, ch, x or z	dress stimulus	add es	dresses stimuluses (Just like nucleus (plural "nuclei"), "stimulus" does not form its plural using the standard rules of English.)
Nouns ending [consonant] o	hero zero tomato	add either s or es (There are no rules for this - you have to know.)	heroes zeros tomatoes
Nouns ending [vowel] o	patio ratio	add s	patios ratios
Nouns ending [consonant] y	story penny	change the y to an i and add es	stories pennies
Nouns ending [vowel] y	donkey chimney	add s	donkeys chimneys
Nouns ending f or fe	dwarf hoof	ves and/or s (There are no rules - you have to know.)	dwarfs hooves or hoofs
Exceptions	man louse	some nouns undergo a vowel or letters change	men lice
More exceptions	salmon sheep	some nouns do not change at all	salmon sheep
Foreign rulings	stimulus medium	some nouns adopt foreign rulings	stimuli media

Read more about English spelling rules.

Why Is There Confusion over the Plural of Stimulus?

Confusion arises over the plural of "stimulus" because its plural form ("stimuli") derives from Latin, and native English speakers are unsure whether "stimuluses," which adheres to the standard ruling for forming plurals, has come into use. Many words with Latin roots (e.g., "nucleus," "stadium," "referendum") have either adopted a standard plural ending alongside the Latin one or have dropped the Latin one entirely (e.g., "referendums" but "referenda" ).

Unlike these words, "stimuluses" is not an accepted plural. You must use "stimuli."

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