Operationalism

the doctrine that the meaning of a proposition consists of the operations involved in proving or applying it

http://www.wordreference.com/definition/operationalism

http://www.thefreedictionary.com/operationalism

http://www.philosophyprofessor.com/philosophies/operationalism.php

Before a study can begin, a fundamental question must be dealt with: How do I measure what I am interested in studying? It is all well and fine to hypothesise a relationship between similarity and attraction, or between a therapeutic program and mental illness, but your ability to provide evidence for such hypotheses will remain at a stand-still until you are able to measure the constructs involved..... (nice long discussion, but has some bits which I think are pretty misleading)