(X)Emacs command, usually bound to M-\. "Delete all spaces and tabs around point." In other words, after M-\, the first word to the left of point gets stuck on the first word to the right. At the beginning of the line (or anywhere before the first word), deletes any indentation. At its end (or anywhere after the last word), deletes any trailing whitespace.
Note the word "horizontal": it does not remove "vertical" whitespace (i.e. newlines).