Definitions of capitalism have been ambiguous and various, not only because the word is not a rigorous economic term but also because it has been used to express many important messages in the context of the 20th century. Generally speaking, it can mean any of the following four concepts : ① liberalism, ② market economy, ③ profit-seeking society, and ④ modern industrial society. This paper attempts to validate these four concepts in turn and make clear the necessary conditions for defining the meaning of capitalism in the context of the 21st century.