I use a third-party console to play SMB. Is it legal to run it on and submit said runs?

If it’s not a whitelisted platform, and I’m assuming it’s not, then no, it is not accepted.

From the rules:

Approved hardware/software (please specify in the run comment):

• Original NES, Famicom or FDS
• Original NES or Famicom with Everdrive or Powerpak
• Wii, Wii U, Switch and 3DS Virtual Console
• FCEUX, Nestopia, Bizhawk, OpenEMU
• NES Classic Edition
• Analogue NT Mini

If it’s any of those, then it’s okay. Otherwise it isn’t allowed.

And to answer the question in the thread title, an emulator is any program or application that allows you to emulate gameplay of video games through a computer without the need of a physical console or cartridge/disk.

i used a hacked wii to practice just dont sumbit runs on it and youll be fine