while

While is a versatile word in English that serves primarily as a subordinating conjunction and as part of an adverbial phrase. In its temporal use, it introduces clauses describing actions occurring simultaneously: "She read a book while he cooked." In a concessive or contrastive sense, it connects two ideas where the second contrasts with the first: "While I understand your reasons, I still disagree." The word can also mark duration or appear in phrases like "for a while."

As a noun phrase, "a while" denotes a period of time; for example, "stay for a while" means to remain for some time. The phrase is common in informal speech and in literature. The etymology traces to Old English hwil, meaning a period or the passing of time, and has developed through Middle English into its modern usage.

In computing, while denotes a control flow construct used to repeat a block of code while a condition holds true. A while loop evaluates its condition before each iteration, so the body may execute zero or more times. C-like languages use syntax such as while (condition) { statements; }, while Python uses "while condition:" followed by an indented block. Do-while loops in some languages perform the loop body first and then test the condition, ensuring at least one execution.

Usage notes: care is needed to avoid ambiguity when "while" contrasts two clauses; in formal writing it is often replaced with "whereas" or rephrased. In programming, improper termination can produce infinite loops; good practice includes updating the condition within the loop or introducing a break condition.