# What's the meaning of "x" in RxD and TxD of UART?

TxD means Transmit Data.

Where the "x" come from?

• Rx is short for receive because I guess someone decided it was easier than writing "receive" all the time. Likewise for Tx/transmit. It's not specific to electronics, you see e.g. pilots saying "pax" for "passengers". Jan 30 '16 at 13:01
• This sort of word etymology question would probably be better on English.SE, in fact there's a related question that probably at least partially answers it: english.stackexchange.com/questions/229929/… Jan 30 '16 at 13:05
• I suspect the word "exchange" has something to do with it. See also PABX and MUX. But X could also mean "cross". Jan 30 '16 at 13:14
• @PeterJ That's an unusually sparse answer for ESE. The use of x as an abbreviation is much older than the technological examples given. Consider Rx (for prescription, which is short for Latin recipe). This is different from the X in Xmas, where X is the first letter of the Greek spelling of Christ (Χριστός). Jan 30 '16 at 14:40
• english.stackexchange.com/questions/37394/… Jan 31 '16 at 11:33

Comes from the 'good old days' of radio when Morse code ruled. Abbreviations abounded (c.f. text messages!), and TX was transmitter or transmit; RX was receiver or receive $$\\cdot\cdot\cdot-\cdot-\$$