At its simplest, optimism is an expectation that good things will happen and that things will work out in the end.

The American Heart Association Scientific Statement 2021 works on the assumption, "Optimism is characterized by having a sense of hopefulness and confidence that things will work out well in the future and anticipating the best possible outcomes."

Researchers from Boston University School of Medicine (BUSM), National Center for PTSD at VA Boston Healthcare System and Harvard T.H. Chan School of Public Health, have found that individuals with greater optimism are more likely to live longer and to achieve “exceptional longevity,” that is, living to age 85 or older.   They use a definition "Optimism refers to a general expectation that good things will happen, or believing that the future will be favorable because we can control important outcomes."

Monash University's Dr Heather Craig and fellow researchers recently wrote, "Optimism, broadly speaking, refers to an individual's hopefulness and/or their belief that something positive or desirable will happen– ‘anticipating good’, and having a positive view of the past, whilst a pessimist expects the worst."

One of the oldest expressions of this optimism came from the middle-ages English anchorite Mother Julian of Norwich who wrote "All shall be well, and all shall be well, and all manner of thing shall be well"