(X)Emacs command, usually bound to M-SPC. "Delete all spaces and tabs around point, leaving one space."

Often useful when reformatting lines in a program: if you join 2 lines, you want to get rid of the second line's extra indentation. delete-horizontal-space does that but leaves nothing; most programming languages need the presence of whitespace to be kept, so you'll need to add at least a space. just-one-space does that for you, simply and elegantly.