Baud rate
The number of symbols per second on a communication link — not necessarily the same thing as bits per second.
The unit is named after Émile Baudot, the telegraph engineer behind Baudot code. On a UART, where each symbol is one bit, baud and bit/s coincide, hence the confusion. Modems, by contrast, packed several bits into each symbol: V.32 ran at 2,400 baud but 9,600 bit/s.
An asynchronous serial link has no shared clock, so each end has to keep the same pace on its own. The tolerance is roughly two to three per cent before bits start being misread. That is why old 8051 boards carry crystals with odd frequencies like 11.0592 MHz: they divide evenly down to 9,600 and 115,200, which 12 MHz does not.