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 (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 multi-tiers architecture
You could 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. However, if your paper is academic, be aware that some use a stricter definition of distributed system.