A reinforcing loop encourages the system to continue in that direction (e.g. a damn starting to leak, as water flows over the damn wall will further erode the wall which leads to more water flowing over causing more erosion).
When the reinforcing loop is undesirable it can be referred to as a viscous cycle...
An example of a positive reinforcing loop is building trust in an organization. As people gain trust (and fear reduces) people are more willing to be trusting and cooperate, that behavior then encourages more of that behavior and so on. A positive reinforcing loop can be called a virtuous cycle.
continue reading: Systems Thinking: Feedback Loops