Marxism and Exploitation

What is exploitation? Marxists make a great deal of hay out of the term. What are they talking about? The dictionary offers a definition that perhaps accidentally includes a subtle but profound distinction. Exploitation is either (a) “to make full use of” or “derive benefit from”, or (b) “to use unjustly”, or “to derive unfair benefit from”. So, on the one hand, we have a neutral term that might even be seen as a positive, in some respects.