Bit-level parallelism is a form of parallel computing based on increasing processor word size. From the advent of very-large-scale integration (VLSI) computer chip fabrication technology in the 1970s until about 1986, advancements in computer architecture were done by increasing bit-level parallelism
Increasing the word size reduces the number of instructions the processor must execute in order to perform an operation on variables whose sizes are greater than the length of the word. (For example, consider a case where an 8-bit processor must add two 16-bit integers. The processor must first add the 8 lower-order bits from each integer, then add the 8 higher-order bits, requiring two instructions to complete a single operation. A 16-bit processor would be able to complete the operation with single instruction)
Historically, all of the early electronic computers were serial computers. The first electronic computer that was not a serial computer -- the first bit-parallel computer -- was the 16-bit Whirlwind - 1951.
Historically, 4-bit microprocessors were replaced with 8-bit, then 16-bit, then 32-bit microprocessors. This trend generally came to an end with the introduction of 32-bit processors, which has been a standard in general purpose computing for two decades. Only recently, with the advent of x86-64 architectures, have 64-bit processors become commonplace.
See also 
- David E. Culler, Jaswinder Pal Singh, Anoop Gupta. Parallel Computer Architecture - A Hardware/Software Approach. Morgan Kaufmann Publishers, 1999. ISBN 1-55860-343-3, pg 15
- "The Two Percent Solution" by Jim Turley 2002