An assessment is a measurement of the knowledge, skill, or performance of a person. Types of assessments include criterion-referenced, norm-referenced, formative, summative, formal, informal, mandated, (state or local), teacher-made, and standards-based assessments.
Attributes common to all assessments include reliability and validity.
A tool, instrument, process, or exhibition composed of a systematic sampling of behavior for measuring a student's competence, knowledge, skills or behavior. An assessment can be used to measure differences in individuals or groups and changes in performance from one occasion to the next. (Handbooks Online)