Origin: Germanic Old English suffix
Tonight has 4 different meanings across 2 categories:
the present or immediately coming night
"We need to finish cleaning up before it gets dark tonight."
The nighttime of the current day or date; this night.
"We should finish our project tonight before going out for dinner."
In plain English: Tonight is the specific evening that is happening right now.
"We will celebrate tonight with a big dinner party."
Usage: Use tonight as a noun to refer specifically to the evening hours remaining in today, such as when saying we will meet later tonight. It is interchangeable with the phrase "this night," but avoid using it for any future dates beyond tomorrow's evening.
During the night following the current day.
"We can't finish the project tonight since I have an early morning flight tomorrow."
In plain English: Tonight means right now during the evening hours.
"We are going to the movies tonight."
Tonight comes directly from the Old English phrase tō niht, which literally meant "to night." The word traveled into Modern English essentially unchanged in form and meaning.