A word is the basic unit of data on which a CPU operates; on many machines it is the size of an instruction. Main memory addresses used to name words, but contemporary practise is to give each byte in main memory its own address.
Words are now usually a power-of-two multiple of an 8-bit byte in size, but in early computers, other word sizes were common. 36 bits was a very popular size on early mainframes, such as the IBM 704 and its descendants, the Honeywell 6000 series, and the PDP-10 family. However, small word sizes were common too, e.g. the 12-bit words of the PDP-8.