2.5D IC is a packaging methodology for including multiple die inside the same package.
One of the big advantages of this approach is that the die do not have to utilize the same process technology, which is a big problem with analog IP at advanced process geometries. Moreover, unlike full 3D-IC, which bonds wafers together, the thermal and electrical properties of 2.5D are easier to manage. Communication between chips is accomplished using interposer technology, silicon photonics, or silicon bridges.
The concept of 2.5D has been around for decades, beginning with multi-chip modules, which packaged multiple die in a single package. System-in-Package, or SiP, uses a similar strategy, and 2.5D sometimes is referred to as SiP, although the interconnect between die has evolved significantly since the SiP concept was first introduced.
3D IC Integration and Packaging
Advances in 3D Integrated Circuits and Systems
Physical Design for 3D Integrated Circuits