onmousedown is the event where a mouse button is first clicked (as opposed to when the button is subsequently released, which is onmouseup). This is not specific to any language, so please also include a language tag if applicable.
Most questions with this tag relate to the HTML DOM event, and you can find further information here: https://en.wikipedia.org/wiki/DOM_events#Events
This event is common across many platforms however so you may need to seek additional references.
