Declarations vs definitions

Question

In C# how does a declaration differ from a definition, i.e.:

1. A class declaration vs a class definition
2. A variable declaration vs definition
3. A met

Answer 1

where the word definition is used, it is used to mean the same thing as declaration

Correct.

The concept of a 'declaration' as a soft/forward definition is needed in C and C++ because of their compilation model. C++ (conceptually) uses single pass compilation, C# is multi-pass. Consider:

class Bar; // declaration: needed in C++, illegal and unnecessary in C#

class Foo  // start of definition, counts as a declaration of Foo
{
    Foo f; // use of declared but still incompletely defined class Foo
    Bar b; // use of declared but still undefined class Bar
}

class Bar  //  definition and re-declaration  
{
}

C++ can't handle the Bar b field without a declaration first. C# can.