I have seen the word "computer system" used in different ways. Sometimes a computer system is defined as consisting of hardware, operating systems, application programs, and the end user. Other times, a computer system is defined as consisting of just the CPU, Memory, and I/O devices.
So, I am wondering if there is one true definition of a 'computer system'. Also, is there a distinction between 'computer system' and 'computer architecture'?
A computer system is any system (a collection of pieces that influence each other) that is able to do (any kind of) computation.
This includes things like (e.g.) the mechanical adding machines from the 1800s. It could also be applied to a group of people sitting in a room who are employed to do computations with pencil and paper, if there's rules (e.g. written procedures for them to follow) that make it systematic.
Of course nobody wants to say "modern (digital electronic) computer system" all the time, so (when the context is clear, which is "almost always" given that alternatives are so infrequently used now) it's natural to shorten it.
However...
It's potentially useful to understand that the English language has no formal standardisation; and when new things are created there's no organisation/committee where you apply for a new word with a new definition. Instead, random people redefine existing words for convenience, and (if these "made up by random people redefinitions" become widespread) the English language evolves. For example, maybe you have a sequence of values that represent characters and want a name for it, so you start with "a string of characters" and before you know it an entire industry is calling it "a string", even though it's nothing like the previous definition of "string" .
With this in mind, you could say that the meaning of "computer system" is evolving towards (or has evolved to?) "modern (digital electronic) computer system".
For "computer architecture", currently there isn't a single definition that has become so common that it can be accepted as "the next true definition". Some people are using it as a synonym for "a standardised programming model" (excluding implementation) while others are using it as "the design of an implementation"; and I'd guess that it's going to take another 20 years or more before either one of these definitions becomes dominant or both become obsolete.
Mostly; I think you're looking for standard definitions for things that random people redefined on a whim.