1. A defect in structure or function. 2. In biostatistics: a mistaken decision, as in hypothesis testing or classification by a discriminant function; or the difference between the true value and the observed value of a variate, ascribed to randomness or misreading by an observer. 3. False-positive and false-negative results in a dichotomous trial. 4. A false or mistaken belief; in biomedical and other sciences, there are many varieties of error, for example, due to bias, inaccurate measurements, or faulty instruments.