Terms such as risk, risk factors, and especially the term cause are inconsistently and imprecisely used, fostering scientific miscommunication and misleading research and policy. Clarifying such terms is the essential first step. We define risk and a risk factor (protective factor) and their potency, set out the conceptual basis of the methods by which risk factors are identified and potency demonstrated, and propose criteria for establishing the status of a risk factor as a fixed or variable marker or a causal risk factor. All definitions are based on the state of scientific knowledge (empirical documentation), rather than on hypotheses, speculations, or beliefs. We discuss common approaches and pitfalls and give a psychiatric research example. Imprecise reports can impede the search for understanding the cause and course of any disease and also may be a basis of inadequate clinical or policy decision-making. The issues in risk research are much too important to tolerate less than precise terminology or the less than rigorous research reporting that results from imprecise and inconsistent terminology.