A property is one kind of member. Others might be constructors, methods, fields, nested types, conversions, indexers etc – depending on the language/platform, of course. A lot of the time the exact meaning of terminology depends on the context.
To give a C#-specific definition, from the C# 3.0 spec, section 1.6.1:
The following table provides an overview of the kinds of members a class can contain.
(Rows for…)
- Constants
- Fields
- Methods
- Properties
- Indexers
- Events
- Operators
- Constructors
- Destructors
- Types
Note that that’s members of a class. Different “things” have different kinds of members – in C#, an interface can’t have a field as a member, for example.