Vitalism is the idea that living organisms possess an inner entity that gives them life. This entity is considered immune from the laws of biology, chemistry, and physics. Depending on geography and audience, this entity is called animal magnetism, chi, energy, ki, prana, a soul, or a vital spark.
Early Vitalists were doing genuine science and reached honest, mistaken conclusions. The field played a role in chemistry, giving us the distinction between organic and inorganic substances. The ancient Greeks incorporated Vitalism into their nascent atomist views. A couple of millenniums later, Hans Driesch discovered he could split a fertilized sea urchin egg and watch its two halves become separate, fully-functioning urchins. He concluded this was being caused by an unseen vital force.
This and similar ideas were dismissed with the advent of genetics, the microscope, and germ theory. A key moment was when Friedrich Wohler synthesized urea from inorganic compounds. This disproved the Vitalist idea that only organisms could make such compounds. Later, Adolph Koble and Marcellin Berhelot synthesized several other organic compounds from inorganic ones. They also demonstrated that larger molecules were composed of smaller ones, as opposed to owing their existence to an esoteric life force. Closer to the modern day, biologists discovered numerous molecular scale mechanisms, such as DNA. For all the research that has been done in this field, nary a trace of vital force has been found.
Vitalism still survives on life support through many forms of alternative medicine. These practices center on the idea that humans are sustained by a force unknown to biology, a force out there somewhere, that somehow in some way sustains us. Chiropractics, acupuncture, and Reiki are all built on the premise of unblocking the flow of an undefined energy. Even more vague are ideas such as aura and chakra. But if a purported entity is immeasurable and unobservable, it is medically irrelevant.
Modern Vitalism also rests on the Appeal to Ignorance. No theory has been developed which fully explains the actions of a single cell, much less a higher form of life, so Vitalists are happy to call chi the missing piece.