A blinker (also called a blinky) is a particular kind of nano-cache container.
The name "blinker" comes from the blinking LED jewelry originally used to create these cache containers. The blinking LEDs are removed or painted over, leaving a compartment for a button-cell battery with a tiny magnet attached. The battery is removed and replaced by a skinny strip of paper rolled up into a scroll (the log sheet), and the magnet was used to attach the blinker to a magnetic object.
Today, ready-made blinkers can be purchased from geocaching suppliers, or you can adapt a flashing magnetic pin or body light yourself.