The dot command ‘.‘ is the equivalent of the C Shell (and Bash) source command. It is specified by POSIX (see dot), and supported by the Bourne and Korn shells (and zsh, I believe).
. somefile
Note that the shell looks for the file using $PATH, but the file only has to be readable, not executable.
As noted in the comments below, you can of course specify a relative or absolute pathname for the file — any name containing a slash will not be searched for using $PATH. So:
. /some/where/somefile
. some/where/somefile
. ./somefile
could all be used to find somefile if it existed in the three different specified locations (if you could replace . with ls -l and see a file listed).
Pedants of the world unite! Yes, if the current directory is the root directory, then /some/where/somefile and ./some/where/somefile would refer to the same file — with the same real path — even without links, symbolic or hard, playing a role (and so would ../../some/where/somefile).