Refers to processors where data are stored in 32-bit registers. Use for questions about hardware or software only if 32-bit architecture is central to the question. For questions comparing 32 vs. 64 bit architecture, use [32-vs-64-bit] instead of the individual [32-bit] and [64-bit] tags.
The maximum integer value that can be stored in 32 bits is 4,294,967,295. Therefore, a processor with 32-bit memory addresses can directly access "only" 4 GB of memory. Which is why it doesn't make sense to put any more memory in a system running a 32-bit operating system.