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In antiquity, all of the enduring celestial bodies that were seen to move relative to the background sky of stars were considered planets. During the Copernican revolution, this definition was altered to objects orbiting around the Sun, removing the Sun and Moon but adding the Earth to the list of known planets. The concept of planet is thus not simply a question of nature, origin, composition, mass or size, but historically a concept related to the motion of one body around the other, in a hierarchical configuration. After discussion within the IAU Commission F2 "Exoplanets and the Solar System", the criterion of the star-planet mass ratio has been introduced in the definition of the term "exoplanet", thereby requiring the hierarchical structure seen in our Solar System for an object to be referred to as an exoplanet. Additionally, the planetary mass objects orbiting brown dwarfs, provided they follow the mass ratio criterion, are now considered as exoplanets. Therefore, the current working definition of an exoplanet, as amended in August 2018 by IAU Commission F2 "Exoplanets and the Solar System", reads as follows: - Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the $L_4$/$L_5$ instability ($M/M_{\rm central}$$<$$2/(25+\sqrt{621}$)$\approx$1/25) are "planets", no matter how they formed. - The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System, which is a mass sufficient both for self-gravity to overcome rigid body forces and for clearing the neighborhood around the object's orbit. Here we discuss the history and the rationale behind this definition.