Deleting with rm did not free up space I deleted a 5gb video using rm video.mp4, but the space hasn't freed.

This was the situation: while encoding a video with FCP X, I ran out of space, I only had 50 MB free. Since in the trash I had some stuff that I wasn't sure that I could delete it yet, I took the terminal and I manually deleted a big video file with rm video.mp4.

The file is gone, but the main problem is that the disk is still full. What I can do to reclaim this space back?

It's normal that manually deleting a file will not free up space?
Probably your file is in use by some process, in this case it's normal. Try to close or kill apps or processes that used that file.