The term derives from the Latin optimum, meaning "best". Being optimistic, in the typical sense of the word, is defined as expecting the best possible outcome from any given situation.[1] This is usually referred to in psychology as dispositional optimism. It thus reflects a belief that future conditions will work out for the best.[2] For this reason, it is seen as a trait that fosters resilience in the face of stress.[3]

Theories of optimism include dispositional models and models of explanatory style. Methods to measure optimism have been developed within both of these theoretical approaches, such as various forms of the Life Orientation Test for the original dispositional definition of optimism and the Attributional Style Questionnaire designed to test optimism in terms of explanatory style.

Variation in optimism and pessimism is somewhat heritable[4] and reflects biological trait systems to some degree.[5] It is also influenced by environmental factors, including family environment,[4] with some suggesting it can be learned.[6] Optimism may also be linked to health.[7]