command-line probably is/should be the canonical tag, with 782 questions. cmd is more Windows-oriented, but is still command-line. cli is about half-and-half, with half on Windows and half on Linux. terminal is about half OS X and half Linux, and terminal.app is mostly (95%) OS X. I propose that:
cmd, cli, terminal, and terminal.app are retagged into command-line.
Alternatively, reading a couple of the answers here, we could:
retag cmd/terminal.app-specific questions to temporary tags, mass retag all the others into command-line, then mass retag cmd and terminal.app questions back.