今晩（こんばん） means tonight or this evening. It can work like a noun or an adverb, so it often appears directly before a verb, as in 今晩行きます（こんばんいきます）, “go tonight.”
The greeting こんばんは comes from 今晩. It originally meant something like “as for this evening…,” with the rest of the phrase dropped over time.
今晩（こんばん） and 今夜（こんや） are often interchangeable. 今夜 can feel a little sharper and is common in news, announcements, and everyday speech. 今日の夜（きょうのよる） is also possible, but it is longer and more literal: the night of today.