For example a test of intelligence should measure intelligence and not something else (such as memory).
Internal and External Validity
A distinction can be made between internal and external validity. These types of validity are relevant to evaluating the validity of a research study / procedure.
Internal validity refers to whether the effects observed in a study are due to the manipulation of the independent variable and not some other factor. In-other-words there is a causal relationship between the independent and dependent variable.
Internal validity can be improved by controlling extraneous variables, using standardized instructions, counter balancing, and eliminating demand characteristics and investigator effects.
External validity refers to the extent to which the results of a study can be generalized to other settings (ecological validity), other people (population validity) and over time (historical validity).
External validity can be improved by setting experiments in a more natural setting and using random sampling to select participants.