A fastener is a hardware device that mechanically joins or affixes two or more objects together. Mechanical fasteners are those that rely on some mechanical principle (not adhesives) and are semi-permanent (unlike clamps). Mechanical fasteners are what we supply. These include nuts, bolts, screws, rivets, washers, threaded inserts, spring steel fasteners (cage-nuts, u-nuts, s-clips, nut retainers, threaded coils, etc.).

Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container, etc. There are also special-purpose closing devices, e.g. a bread clip. Items like a rope, string, wire (e.g. metal wire, possibly coated with plastic, or multiple parallel wires kept together by a plastic strip coating), cable, chain, or plastic wrap may be used to mechanically join objects; but are not generally categorized as fasteners because they have additional common uses. Likewise, hinges and springs may join objects together, but are ordinarily not considered fasteners
because their primary purpose is to allow articulation rather than rigid attachment. Fasteners used in these manners are often temporary, in that they may be fastened and unfastened repeatedly.

Some types of woodworking joints make use of separate internal reinforcements, such as dowels or biscuits, which in a sense can be considered fasteners within the scope of the joint system, although on their own they are not general purpose fasteners. Furniture supplied in flat-pack form often uses cam dowels locked by cam locks, also known as conformant fasteners.

Other alternative methods of joining materials include: crimping, welding, soldering, tapingbrazing, , gluing, cementing, or the use of other adhesives. The use of force may also be used, such as with magnets, vacuum (like suction cups), or even friction.