Because "stimulus" is a Latin word. In Latin, most words ending in "-us" form the plural in "i". They keep this in English.
The plural for stimulus is stimuli.
"Stimulus" is singular; "stimuli" is plural.
Stimulus can be both singular and plural depending on its usage. As a singular noun, it refers to a single influence or prompt. As a plural noun, stimuli is used to refer to multiple influences or prompts.
stimulus = singlestimuli = plural
The noun stimulus is singular. The plural is stimuli - as in "The patient did not respond, even to repeated stimuli". Note that 'stimulus' is becoming increasingly used as an adjective - stimulus measures, a stimulus package - since the credit crunch of 2009.
Stimulus (singular) and Stimuli (plural)
The plural of stimulus is stimuli.
The foreign plural for "stimulus" is "stimuli."
Stimuli is already the plural of stimulus.
Stimuli is the plural of stimulus.
Stimuli is the plural of stimulus.
the difference is that, stimuli is the plural of stimulus; that is stimuli is feelings while stimulus is feeling.