What is 16 bit in different contexts ?

Refers to the number of bits that can be processed
or transmitted in parallel, or the number of bits used
for single element in a data format. The term is often
applied to the following:


microprocessor: indicates the width of the registers. 
A 16-bit microprocessor can process data and memory
addresses that are represented by 16 bits.

bus : indicates the number of wires in the bus. 
A 16-bit bus transmits 16 bits in parallel.

graphics device, such as a scanner or digital camera :
specifies the number of bits used to represent each pixel.

operating system: refers primarily to the number of
bits used to represent memory addresses. Windows 3.x
is a 16-bit operating system, whereas Windows 95 and
Windows NT  are  32-bit operating systems.


expansion board: refers to how much data can be sent
to and from the card in parallel. 8-bit cards are
sometimes called half-size cards whereas 16-bit cards
are referred to as full-size cards.

source: https://www.webopedia.com/TERM/1/16_bit.html