I like Hernando de Soto’s answer from his book, The Mystery of Capital: Why Capitalism Triumphs in the West and Fails Everywhere Else (p. 41):
To unravel the mystery of capital, we have to go back to the seminal meaning of the word. In medieval Latin, “capital” appears to have denoted head of cattle or livestock, which have always been important sources of wealth beyond the basic meat they provide. Livestock are low-maintenance possessions; they are mobile and can be moved away from danger; they are also easy to count and measure. But most important, from livestock you can obtain additional wealth, or surplus value, by setting in motion other industries, including milk, hides, wool, meat, and fuel. Livestock also have the useful attribute to reproduce themselves. Thus the term “capital” begins to do two jobs simultaneously, capturing the physical dimension of assets (livestock) as well as their potential to generate surplus value. From the barnyard it, it was only a short step to the desks of the inventors of economics, who generally defined “capital” as that part of a country’s assets that initiates surplus production and increases productivity.