(1) See
ping.
(2) The transmitter in an aviation flight recorder (the airplane's black box) that emits a 37.5 kHz signal each second until the battery is drained. Officially an "underwater locator beacon" (ULB), the pinger is activated when immersed in water.
(3) (Pinger Inc.) The publisher of Textfree, a texting and voice application for PCs, Macs, iPhones and Android devices.