From a 1985 article in the Public Health Reports:
Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research.
Authors: C J Caspersen, K E Powell, and G M Christenson
"Physical activity," "exercise," and "physical fitness" are terms that describe different concepts. However, they are often confused with one another and the terms are sometimes incorrectly used interchangeably. This paper proposes definitions to distinguish them. Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure. The energy expenditure can be measured in kilocalories. Physical activity in daily life can be categorized into occupational, sports, conditioning, household, or other activities. Exercise is a subset of physical activity that is planned, structured, and repetitive and has as a final or an intermediate objective the improvement or maintenance of physical fitness. Physical fitness is a set of attributes that are either health- or skill-related. The degree to which people have these attributes can be measured with specific tests. These definitions are offered as an interpretational framework for comparing studies that relate physical activity, exercise, and physical fitness to health.