A New Dictionary

Logic is the study of the principles of valid inference and demonstration. The word derives from Greek λογική (logike), fem. of λογικός (logikos), "possessed of reason, intellectual, dialectical, argumentative", from λόγος (logos), "word, thought, idea, argument, account, reason, or principle".

"Logic is the branch of philosophy that treats of forms of thinking."

"The science or art of exact reasoning, or of pure and formal thought, or of the laws according to which the processes of pure thinking should be conducted; the science of the formation and application of general notions; the science of generalization, judgment, classification, reasoning, and systematic arrangement; correct reasoning." ~ A treatise on Logic