1) A stable and, usually, recoverable performance upon which effects of experimental variables are superimposed (e.g., a drug effect may be expressed as the change produced in baseline response rate by a dosage of the drug). The term is also used occasionally to refer to the horizontal starting position (zero responses) of a cumulative-recorder pen. 2) The base rate of behavior against which an experimental manipulation is measured. 3) Repeated measures of the strength or level (e.g., frequency, intensity, rate, duration, or latency) of behavior before an experimental variable (treatment, intervention, or procedure) is introduced. Baseline measurements are continued until performance has stabilized and can be used as a basis for assessing the effects of the experimental variable. BASELINE 4) An abbreviated reference to a baseline condition or to the data from a baseline phase. 5) The level of a particular behavior before it was changed. Baseline is the same thing as operant level, starting level, or pretreatment level. If these are measured in rate, they are also equivalent to base rate. 6) The phase of an experiment or intervention in which the behavior is measured in the absence of an intervention. 7) A period during which the target behavior is recorded, but no attempt is made to modify it.