What is the difference between SoC (System-on-chip) and SiP (System-on-Package)

A System on Chip (SOC) is a single chip that incorporates all of a system's typical functionalities into a single chip. This could include one or more processor cores (single, dual, quad, or octa), memory subsystems (such as DDR controllers), IO subsystems (such as PCIE, SATA, USB, etc.), or other functional logic or IP that is similar and is produced as a single integrated circuit device.

A System-In-Package (SIP) is a higher level of integration that incorporates multiple dice into a single package. In the past, multiple specialized IC devices were required to be connected and assembled on a PCB; however, this level of connectivity is now integrated into the package itself. In order to join multiple dies together, both horizontally and vertically, a number of packaging methods are utilized. 2D and 3D) SOC is a design trend that has become more prevalent over the past decade or so in the smartphone and mobile computing era. More integrations will be possible with SIP in the future.

I just read an interesting article about SIP: the Apple Watch's S1 chip integrates more than 30 chips, including several passive components.