(noun) an explanation of the fundamental reasons or basis for something
"The judge crafted a compelling rationale for the court's decision."
justification: "The legal justification for the statute was that the legislature had not previously addressed the issue of hate crimes."
grounds: "Increases in airport waiting times are justified on the grounds of national security."
Common phrases
legitimate rationale: "Is there a legitimate rationale for such regulation?"
the rationale behind: "He explained the rationale behind the tests."
develop a rationale: "They developed a rationale to explain the change in prices from year to year."

REASON  (n.)
1.----The basis or motive for an action, decision, or conviction.
2.----A declaration made to explain or justify action, decision, or conviction: inquired about her reason for leaving.
3.----An underlying fact or cause that provides logical sense for a premise or occurrence: There is reason to believe that the accused did not commit this crime.
4.----The capacity for logical, rational, and analytic thought; intelligence.
5.----Good judgment; sound sense.
6.----A normal mental state; sanity: He has lost his reason.
7.----Logic. A premise, usually the minor premise, of an argument.

n. An emotion, desire, physiological need, or similar impulse that acts as an incitement to action. A motif in art, literature, or music.
adj. Causing or able to cause motion: motive power. Impelling to action: motive pleas. Of or constituting an incitement to action.

n. A recurrent thematic element in an artistic or literary work. A dominant theme or central idea. Music. A short rhythmic or melodic passage that is repeated or evoked in various parts of a composition. A repeated figure or design in architecture or decoration.