Memory page is a range of consecutive addresses in the virtual address space whose size is a power of two. Those virtual addresses are translated in a corresponding range of consecutive physical addresses. The memory referenced by such a range is called a page.
Size of memory page is architecture and operating system dependant. On an x86 and x86_64 hardware, memory page is equal to 4KB. On an Intel Itanium (IA64) memory page size is 16KB.