Constitutionality is the condition of acting in accordance with an applicable constitution;[1] the status of a law, a procedure, or an act’s accordance with the laws or guidelines set forth in the applicable constitution. When one of these (laws, procedures, or acts) directly violates the constitution it is unconstitutional. All the rest are considered constitutional until challenged and declared otherwise.

As an overview, you may read more at: http://en.wikipedia.org/wiki/Constitutionality

1.^ http://www.merriam-webster.com/dictionary/constitutionality “Webster On Line”