Cross-platform play
Cross-platform play, cross-platform multiplayer or simply cross-play is the feature offered by some games where users can play a game online together regardless of the platform their game is.
Such functionality was first to be pioneered in NES, SNES and Sega Genesis consoles with the Teleplay Modem which would allow owners of these 3 consoles to play together in games developed with this modem in mind, such as BattleStorm, a tank combat simulator developed by the author of said modem, Keith Rupp, who formed his company Baton Technologies in a small office in an industrial park near Skyharbor Airport in Phoenix. Studios like Electronic Arts and Sierra were in talks with Baton Technologies' Keith to release cross-platform games but decided against it as the modem and the games weren't licensed, obtaining a license from Sony or Nintendo was hard, and due to pressure by Nintendo of America most major retailers refused to exhibit unlicensed titles.
As such, the modem was cancelled as Sega and Nintendo didn't license it and the games developed it, all in spite of Baton Technologies actually having fully-working 2400-bit-per-second prototype modems (with a standard AT keyboard connector for potential internet access) on hands along witn 3 game prototypes were actually made to demonstrate the modem's capabilities.
Another such ill-fated accessory was the unreleased Edge 16 Modem developed by AT&T in partnership with PF Magic and Sega of America. The Edge 16 Modem was supposed to bring cross-play to Sega Genesis/Mega Drive and Panasonic 3DO. Unlike Baton Technologies, AT&T actually managed to convince Sega to license them instead, but they never delivered, and the unfulfilled promise was actually the reason for the failure of the Teleplay Modem (investors simply pulled of the project once they heard of AT&T's licensed competing project).
Later, a company called Catapult licensed by both Nintendo and Sega came to the market with their XBAND Modem for SNES and Sega Genesis. Similar in many aspects, it actually became the first to officially enter the market and enjoyed a limited success but ultimately failed, and no games were actually supported cross-play, the only multiplatform piece of software was Xmail", an e-mail client that came bundled with the modem.
However, the Sega Mega Drive version of SanSan (an online service for Go, a Japanese board game) released in 1994 for the Mega Modem did offer multiplayer where SMD players could play against PC players. It was very niche, thoughm
The cross-play idea would with the Sega Dreamcast. Very revolutionary for its time, it pushed cross-play between a console and a home computer to mainstream with the release of Quake 3 Arena, allowing to set-up a joint PC/Dreamcast server.
And you can actually still do it! All you need is an unpatched game for Windows/Linux/macOS, the 1.16n patch and the official Dreamcast map pack.
Another case was 4x4 Evo which supported cross-play between the Dreamcast, Windows and macOS versions (the PS2 version had its own separate servers); it, too, can be experienced these days with the server hosted by Dreamcast-Talk.
Additionally, Millionaire Fighting 2001 (the Japanese version of Capcom vs. SNK 2: Mark of the Millennium 2001) was first of its kind, the 1st instance of cross-play between directly competing consoles as it offered online matches between Dreamcast and PS2 players in Japan, which was previously unheard of. The Dreamcast version was only released in Japan. Correspondingly, only the Japanese PS2 version offered cross-play, with other international PS2 releases (including Asian non-Japanese) having removed that unique capability.
In some cases, cross-play can be still be experienced on emulators if there is an official server software client is released (or an unofficial reverse-engineered alternative available) for the relevant game in question and if the relevant emulator(s) support(s) the online functionality of the emulated system(s) (a console or a home computer).