What is the difference between declaring a variable and defining a variable in c language

What is the difference between declaring a variable and defining a variable?

Declaration of a variable in C hints the compiler about the type and size of the variable in compile time. Similarly,
declaration of a function hints about type and size of function parameters. No space is reserved in memory for
any variable in case of declaration.

Example: int a;

Here variable 'a' is declared of data type 'int'
Defining a variable means declaring it and also allocating space to hold it.
We can say "Definition = Declaration + Space reservation".

Example: int a = 10;

Here variable "a" is described as an int to the compiler and memory is allocated to hold value 10.