certain

Certain is an English word that functions as an adjective and as a determiner, denoting either reliability or a subset of things. As an attributive determiner, it indicates that some but not all members of a category are being referred to. For example: "Certain species migrate" or "There are certain risks involved." When used with an article, "a certain," the phrase marks a particular but unnamed individual or item: "She spoke with a certain man at the conference." In general, it signals specificity without identification.

As a predicative adjective, certain means definite or guaranteed: "The outcome is certain" or "He is certain to arrive." It can also express confidence, as in "I am not certain about that." The nuance compared to "sure" is subtle: certain emphasizes objective likelihood or established fact, while sure emphasizes subjective confidence.

The word "certain" is derived from Latin certus 'fixed, settled,' via Old French certain, and entered English in the Middle Ages. It is often used in formal or analytic contexts and in set phrases such as "for certain" or "certainly," the latter being the adverbial form.

In contemporary usage, "certain" is common in both spoken and written English, but preference may vary by register. It is also used in legal, philosophical, and scientific writing to denote definite facts, as in "certain measurements" or "certain conditions."