1. To prepare to read or write a file. This usually involves checking whether the file already exists and that the user has the necessary authorisation to read or write it. The result of a successful open is usually some kind of capability (e.g. a Unix file descriptor) - a token that the user passes back to the system in order to access the file without further checks and finally to close the file.
2. Abbreviation for "open (or left) parenthesis" - used when necessary to eliminate oral ambiguity. To read aloud the LISP form (DEFUN FOO (X) (PLUS X 1)) one might say: "Open defun foo, open eks close, open, plus eks one, close close."
3. Non-proprietary. An open standard is one which can be used without payment.
4. open interval.