What is the singular of stimu?

The word "stimu" is not a recognized English word. It's likely a misspelling or a word from another language.

Possible Correct Spellings:

* Stimulus: This is the singular form of "stimuli" and refers to something that causes a reaction or response.

* Stimulant: This refers to a substance that increases activity.

Please provide more context or information about where you encountered the word "stimu" so I can better understand what you're looking for.