I'd define "bloat" as functionality (as in: program code) present on my system that I cannot imagine ever needing to use.

There will never be a system that is perfectly tailored to my needs because there will always be some piece of functional code that I have no intention of using. Therefore, any system is "bloated" and it's a question to which degree it is "bloated".

The degree depends on which kind of resources the "bloat" uses and how much of it. The more significant the resource usage, the more significant the effect of the "bloat". The kind of resource is used defines how critical some amount of usage is. 5% Power, CPU, IO, RAM or disk usage have varying degrees of criticality for instance.

Some examples:

This system has a calendar app installed by default. I don't use it, so it's certainly bloat but I also don't care because it's just a few megs on disk at worst and that doesn't hurt me in any way.

Firefox frequently uses most of my RAM and >1% CPU util at "idle" but it's a useful application that I use all the time, so it's not bloat.

The most critical resource usage of systemd (pid1) on my system is RAM which is <0.1%. It provides tonnes of essential features required on a modern system and therefore not even worth thinking about when it comes to bloat.

I just noticed that mbrola voices sneaked into my closure again which is like 700MiB of voice synthesis data for many languages that I don't have a need for. Quite a lot of storage for something I don't ever need. This is significant bloat. It appears Firefox is drawing it in but it looks like that can be disabled via an override, so I'll do that right now.