Short Answer — Terminal which allows customers or employees to get information and/or conduct a transaction, without the assistance of a person.
Long Answer — The original use of the word “kiosk” started as the town square notice board for the community to post notices in Turkey. The usual reference in Wikipedia will call out Persia as the originating language for the word. What began as common ground notice posting location matured into RMUs (Remote Merchandising Units) that you see in malls or wherever. With the advent of the common internet they took on their electronic iteration in the late 90s.
For the masses, it started with airline check-in terminals, ATMs and photo kiosks (from Kodak and Fujifilm).
In modern times (2020) you have portable charging units for mobile phones, patient check-in kiosks, all types of ticketing, DMV license renewal kiosks, all types of bill payment (via credit card, cash, or check even). Smart city implementations of digital wayfinding with large format screens are common. Drive thru at restaurants, curbside pickup, and all types of automated locker pickup and dropoff. The list goes on. Recently we were asked if electric charging stations were kiosks. Short answer is yes. See the FAQ question here regarding those for the extended answer.
The big driver these days is the wide use of the mobile phone and the new emerging payment interaction methods. It used to be a paper check or cash. Not anymore… Those have since been reduced to contingency-only. There are arguments revolving around the unbanked and underbanked for preserving those methods but the data shows most of the underbanked/etc actually rely more heavily on a mobile (and a digital transaction). They are more likely to get a money card than a written check (or cash).
And then there is the questions of sanctions such as laws. For example, you may not consider a burrito a sandwhich right? However, when it comes to nutrition and food safety the FDA considers a burrito as a sandwich. Sounds silly for sure but it illustrates equivalents. ATM manufacturers like to say their machines are ATMs. Is an ATM machine any different, in abstract sense, from a vending machine, or a reverse payment terminal?
For more information here is an extended article on kioskindustry