The Fixing Charm is a spell that magically fastens one thing to another.
This spell may be temporary, unlike the Permanent Fixing Charm.
References from the canon
Elveira Elkins wrote to the Daily Prophet Problem Page because she could not make a simple Fixing Charm 'stick'. The correspondent who dealt with everyday magical problems diagnosed that the caster was allowing her attention to wander whilst 'charming', and recommended using nails instead (DP).
Latin fixus "fixed and immovable"