I found two clear definitions for "tyre":
- The name of a port city in Lebanon
- The British spelling of the rubber ring that goes around a car's wheel. (i.e. the 'tire')
I personally identify n-tier architecture as being applications that are spread across different physical spaces. Most applications are 2-tier, the applications runs on one system and it connects to a database on another.
I think what most people interpret n-tier to be is what I call n-layer. You can have several logical layers to your application, but in many (and indeed most) cases, the layers are running on the same tier. However, building well designed layers will permit you to move them to different tiers.