Your system has a 3 tier architecture, since it has the following components:
- a front-end (presentation tier)
- a back-end (application tier)
- a database engine (data tier)
and each component could run on a different computer.
If you'd have more than 3 layers, you'd have an N-tier or multitier architecture, provided that each layer communicates only with the previous and the next layers.
You could also say that it's a distributed system, since it is one system made of components that are distributed on several computing nodes and communicate between them. The distributed system is a more general architecture than the multitier, since it doesn't have constraints on how the components communicate between them (e.g. each component could discuss with each other). However, if your paper is academic in the field of computing, be aware that some use a stricter definition of distributed systems.