A page is the smallest unit of memory the MMU can manage and map. 4KB is the most common size, but some architectures like AARCH64 optionally also support 64KB pages. VIMIX only supports 4KB pages but tries not to have implicit dependencies on this value.
Kernel code uses the PAGE_SIZE define from kernel/page.h.
User space applications can figure out the page size by calling sysconf:
#include <unistd.h>
long page_size = sysconf(_SC_PAGE_SIZE);Overview: memory management
Related: kernel memory map | process memory map | page