Whether a test or study actually measures what it aims to measure. Internal validity shows whether a study or test is appropriate for the question, for example, whether a study of exercise among gym members measures the amount of exercise people do at the gym, not simply whether people join. External validity is the degree to which the results of a study hold true in non-study situations, for example, in routine NHS practice. It may also be referred to as the generalisability of study results to non-study populations. For example, the external validity of a study that took place in Spain may be questioned if the results were to be applied to people in Australia.

External validity:
The degree to which the results of a study hold true in non-study situations, for example in routine NHS practice. May also be referred to as the generalisability of study results to non-study populations. For example, the external validity of the study that took place in Spain may be questioned if the population the results were applied to was people in Australia.

Internal validity:
A measure of how well a research study has been designed. That is, the extent to which the cause-and-effect relationships in a study are true for the people and conditions of the study.

See also, Validity, reliability and generalisability from Healthknowledge.org.uk