In a study, validity is the degree to which the conclusions that the researchers make can be considered to be 'true', based on how well the study was designed and how well it matched 'real life' situations.
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.
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.