any stimulating information or event; acts to arouse action
"The sudden noise acted as a stimulus that startled the dog into running away."
Any external phenomenon that has an influence on a system, by triggering or modifying an internal phenomenon.
"The sudden loud noise acted as a stimulus that triggered the dog's startle reflex."
In plain English: A stimulus is anything that causes you to react, feel something new, or change your behavior.
"The sudden noise acted as a stimulus that made her jump out of her chair."
Usage: In casual conversation, speakers often misuse the plural as "stimuluses," but the standard form is "stimuli." This term is also frequently confused with "stigma" due to their similar spelling and pronunciation.
The word comes from the Latin stimus, which originally meant a goad or a sharp stick used to prod an animal. It traveled into English with this sense of something that provokes action or response.