A risk is a condition that can result in a loss. Risk can only be controlled in many scenarios but not eliminated completely. Defect normally converts to a risk. For instance let’s say you are developing an accounting application and you have done wrong tax calculation. There is a huge possibility this will lead to risk of company running under loss. But if this defect is controlled then we can either remove this risk completely or minimize it. Below diagram shows how a risk is converted to a defect and with proper testing how it can be controlled.
Defect and risk relationship