In a traditionally drawn decision tree a circle represents?

Short answer: a circle represents a "chance node," meaning a chance to which an Expected Monetary Value (EMV) may be assigned to calculate the most likely pay-off.

Explanation: according to the PMBOK Guide, 4th Ed. (pgs. 298-299), the inputs to this node are scenario probability and the EMV (expected monetary value) assigned to that chance node, from the decision node (denoted as a square on a decision tree). A chance node leads to a "net path value," otherwise known as a computed value or solution set (represented by a triangle on a decision tree).

A decision tree of this type is one of the tools used to determine quantitative risk analysis used to reach a computed optimal solution using probability distributions as a basis.