Transporter armband
A transporter armband is a portable device worn on the arm that allows the wearer to remotely activate a transporter system, causing it to "beam up" the wearer. By the time of Star Trek: Nemesis, these devices had been miniaturized to the point that they were as small as a cuff link or lapel pin.
These devices sometimes allow transport through inhibitor fields or other interference, suggesting they may have pattern enhancement technology in addition to remote activation. They can be linked to any transporter system, including the miniature units located on shuttlecraft.
Transporter armbands are typically used by small away teams, operating from a shuttlecraft or other small ship, who may need to leave their vessel unmanned during their mission.
Usage
Worf activates his armband...
...and transport commences.
Unusual Applications (Warning: Technobabble)
Transporter armbands generate a subspace field and incorporate a "phase discriminator" (lending support to the idea that they have a pattern enhancement function). In TNG "Timescape", crewmembers were able to modify the fields generated by these armbands to protect themselves from a "temporal fragment" in which time flowed at a far different rate.