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.