to ascribe (one's acts, opinions, etc.) to causes that superficially seem reasonable and valid but that actually are unrelated to the true, possibly unconscious and often less creditable or agreeable causes.

to remove unreasonable elements from.

to make rational or conformable to reason.

to treat or explain in a rational or rationalistic manner.

Mathematics . to eliminate radicals from (an equation or expression)

to rationalize the denominator of a fraction.

Chiefly British . to reorganize and integrate (an industry).

to invent plausible explanations for acts, opinions, etc., that are actually based on other causes

He tried to prove that he was not at fault, but he was obviously rationalizing.

to employ reason; think in a rational or rationalistic manner.

noun

explanation , justification , rationale , reason