which

which returns the first matching command(s) in $PATH

$ which echo
/bin/echo

Useful Options / Examples

which -a [command(s)]

which can be used with the flag -a to display all matching commands in $PATH, not just the first

$ which -a less
/usr/bin/less
/bin/less

which [program]

which can be used with certain programs

$ which nano
/usr/bin/nano

which [commands]

which can be with multiple input commands at once

$ which less nano
/usr/bin/less
/usr/bin/nano

Putting it all together

$ which -a less nano
/usr/bin/less
/bin/less
/usr/bin/nano
/bin/nano

The command will display all paths in the order they are found. In this case, all locations of less are listed in order first and then the same for nano