Is the opposite word of "indent" is "outdent", or is it "unindent"?
Corresponding to "Tab" and "Shift Tab" in most editors.
In that case you'll want unindent, as it reverses the indent action, and as outdent refers to a (usually left-) hanging block of text.
The opposite of indent is outdent, according to Christopher Hoot, graduate of Yale University and graphic design instructor at the University of Akron.
Outdent is modelled on indent, replacing in with its opposite, out.
It refers to multident -> multiple “dents” in a paragraph.
Verb outdent is to indent negatively, bring towards the margin.
You can also use de-indent.