A calling frequency within an amateur radio band (typically for frequencies above 50 MHz) that amateur radio operators have agreed to reserve for finding other amateurs and then moving off to another frequency to continue their QSO.
Typically, while doing mobile operation in a remote area or some place you're unfamiliar with, tuning to a National Simplex frequency in your band of choice and calling CQ will get you a response more often than any other simplex frequency.
National Simplex on the major VHF/UHF and SHF amateur radio bands are as follows:
A few decades ago, the two meter National Simplex frequency was 146.940 MHz. It changed to 146.52 with the new ARRL band plans, there are, however a very large number of repeaters established with 146.94 MHz as their output, and 146.34 as their input, and 146.94 tuning crystals are relatively easy to find at hamfests.