Now, we provide the definitions of A, B and C.
In the above sentence, should I put an
s for word
definition or not?
The answer isn't really about whether you're listing items or not, but about whether there is one definition or more than one definition. If all of the terms have one definition, then the singular, definition, is appropriate. If there is more than one definition, then it has to be the plural, definitions.
For example, you might say
The definition of cover, conceal, or hide is "to keep out of sight".
if they all mean the same thing in that context, but
The definitions of plaintiff and defendant are as follows...
because plaintiff and defendant have different definitions.