INPROGRESS is a file extension native to files generated by Time Machine, a software tool for creating data backups, available for OS X 10.5 and later.

Time Machine uses INPROGRESS files to store temporary backup copies of files. Once the archivization process is complete, INPROGRESS file is removed automatically. However, if the archiving process fails for some reason, INPROGRESS file must be deleted manually. After system restart files for archiving can be indexed again for Time Machine to create backup copy of those files.