The United States National Aeronautics and Space Administration (NASA) is not a medical term or organization, but rather the civilian space agency of the U.S. federal government. It is responsible for the nation's civilian space program and for aeronautics and space research. However, NASA has made significant contributions to medicine and health through its research and technological developments. For example, it has developed medical technologies for use in space that have also been applied to patient care on Earth, such as improved imaging techniques and telemedicine systems. Additionally, NASA's studies of the effects of space travel on the human body have led to advances in understanding and treating various health conditions, including bone loss, muscle atrophy, and radiation exposure.

An astronaut is a professional who is trained and competent to travel in space outside of the Earth's atmosphere. The term "astronaut" is commonly used to refer to individuals from the United States, while the terms "cosmonaut" and "taikonaut" are used for those from Russia and China, respectively.

Astronauts undergo rigorous training and physical examinations to ensure they are in good health and can handle the demanding conditions of space travel. They may perform various tasks during their missions, including scientific research, operating equipment, maintaining the spacecraft, and communicating with mission control on Earth.

The first human astronaut was Yuri Gagarin, a Russian cosmonaut who became the first person to orbit the Earth in 1961. Since then, thousands of people from various countries have become astronauts and have contributed to our understanding of space and the universe.

"Space flight" is not a term that has a specific medical definition. However, in general, it refers to the act of traveling through space, outside of Earth's atmosphere, aboard a spacecraft. This can include trips to the International Space Station (ISS), lunar missions, or travel to other planets and moons within our solar system.

From a medical perspective, space flight presents unique challenges to the human body, including exposure to microgravity, radiation, and isolation from Earth's biosphere. These factors can have significant impacts on various physiological systems, including the cardiovascular, musculoskeletal, sensory, and immune systems. As a result, space medicine has emerged as a distinct field of study focused on understanding and mitigating these risks to ensure the health and safety of astronauts during space flight.

I'm sorry for any confusion, but "United States" is a geopolitical entity, specifically the name of the country consisting of 50 states, and it is not a medical term or concept. Therefore, I can't provide a medical definition for it. If you have any questions related to health, medicine, or biology, I would be happy to try to help answer those!

National Health Insurance (NHI) in the United States does not refer to a specific federal program, but rather it is often used to describe the concept of universal healthcare financing, where all residents have access to necessary healthcare services, and the costs are shared among the entire population.

However, the closest equivalent to NHI in the US is Medicare, which is a federal social insurance program that provides health insurance coverage to people aged 65 and older, some younger people with disabilities, and people with end-stage renal disease. It is not a true NHI system because it does not cover all residents of the country.

Therefore, there is no widely accepted medical definition of 'National Health Insurance, United States' in the context of an actual existing program or policy.