This is a first suggestion for tag wiki excerpt given the answers of @gung and @Glen_b.
Validity is the extent to which a concept, conclusion or measurement is well-founded and corresponds accurately to the real world. The validity of a measurement tool is considered to be the degree to which the tool measures what it claims to measure. DO NOT use this tag for "validation" model purposes.
Validation is the process of deciding whether the numerical results quantifying hypothesized relationships between variables, obtained from regression analysis, are in fact acceptable as descriptions of the data.
DO NOT use this tag for asking if it is valid ("validity") to use a certain test, measurement or analysis. Please, manifest the suggestion with upvotes, downvotes, additional comments, additional answers (we have right now 17-July-2013: question -1, @gung +9, Glen_b +6).
Edit 2 (status-completed):
The tag wiki excerpts were edited and approved according to modifications suggested by @gung and @Glen_b. See them here and here.
Further improvements can be suggested directly on the tag wiki excerpts pages or with the opening of a new thread.