Principle: A fundamental truth or law as the basis of reasoning or action. Principles in the context of sustainable forest management are seen as providing the primary framework for managing forests in a sustainable fashion. They provide the justification for criteria, indicators and verifiers. Consider that principles embody human wisdom. Wisdom is defined as a small increment in knowledge created by a person's (group's) deductive ability after attaining a sufficient level of understanding of a knowledge area. Wisdom therefore depends on knowledge.
Criterion: A principle or standard from which something is judged. A criterion can therefore be seen as a "second order" principle, one that adds meaning and makes the principle operational without itself being a direct measure of performance. Criteria are the intermediate points to which the information provided by indicators can be integrated and where an interpretable assessment crystallizes. Principles form the final point of integration. In addition to considering criteria to be second-order principles, treat them also as reflections of knowledge. Knowledge is the accumulation of related information over a long period of time. It can be viewed as a large-scale selective combination or union of related pieces of information.
Indicator: An indicator is any variable or component of the forest ecosystem or management system used to infer the status of a particular criterion. Indicators should convey a "single meaningful message". This "single message" is termed information. It represents an aggregate of one or more data elements with certain established relationships.
Verifier: Data or information that enhances the specificity or the ease of assessment of an indicator. The fourth level of specificity, verifiers provide specific details that would indicate or reflect a desired condition of an indicator. Verifiers add meaning, precision and usually also site-specificity to an indicator. They may define the limits of a hypothetical zone from which recovery can still safely take place (performance threshold/target). On the other hand, they may also be defined as procedures needed to determine satisfaction of the conditions postulated in the indicator concerned (means of verification).
Source: The CIFOR Criteria and Indicators, Toolbox Series No. 2.