I'm a big proponent of saying command line when you mean to get a shell running. Terminal app is just one app used to get to the command line, so I strongly prefer tagging those questions with command-line as opposed to terminal on this site. I believe this is a little different than Apple's usage since when they tell people to use the command line, they are supporting their tool as opposed to having people need to connect Terminal with being asked to do something on the command line. (or bash / zsh / fish / whatever shell you prefer).
For Apple's usage I look for the latest Apple Publication Style Guide - the latest I know of is:
Finder Use the with Finder (exception to the rule to not use the with application names).
So you would open the Finder or interact with a Finder window. You wouldn't open Finder if you wanted to adhere to the published style guide.
terminal A device or software for sending commands to a computer. Capitalize when referring to the Terminal application.