The imitation of one device or system by another is?
The imitation of one device or system by another is Emulation.
So while it doesn't make sense to use emulators without ROMs, it is legal. Just like it is legal to have an empty beer bottle if you're under 16 but not have alcohol. There is no 24/48 hour exemption. Linking to copyrighted content, hosting it and downloading it is always illegal.
In computing, an emulator is hardware or software that enables one computer system (called the host) to behave like another computer system (called the guest). An emulator typically enables the host system to run software or use peripheral devices designed for the guest system. Emulation refers to the ability of a computer program in an electronic device to emulate (or imitate) another program or device. Many printers, for example, are designed to emulate Hewlett-Packard LaserJet printers because so much software is written for HP printers. If a non-HP printer emulates an HP printer, any software written for a real HP printer will also run in the non-HP printer emulation and produce equivalent printing. Since at least the 1990s, many video game enthusiasts have used emulators to play classic (and/or forgotten) arcade games from the 1980s using the games' original 1980s machine code and data, which is interpreted by a current-era system.