In a monoreactive harmonic oscillator, inert elements can make free sinusoidal oscillations, which are accompanied by the transformation of one inert element kinetic energy into the kinetic energy of another inert element. In this condition the energy of the first inert element is zero. At the same time, the energy of the second element has the maximum value. At the next moment of time, the first element acquires acceleration due to the kinetic energy of the second element, the speed of which begins to decrease. In a classical oscillator, free sinusoidal oscillations are accompanied by an exchange of energy between its elements having the opposite reactivity character. In a spring pendulum, the potential energy of an elastic element is transformed into the kinetic energy of an inert element and vice versa. These elements have the opposite character of reactivity. In an electric oscillatory circuit, the energy of the coil magnetic field is transformed into the energy of the condenser electric field and vice versa. These elements also have the opposite character of reactivity. There are also oscillators in which free sinusoidal oscillations are accompanied by the transformation of the kinetic energy of an inert element or the potential energy of an elastic element into the energy of the coil magnetic field or the energy of the capacitor electric field and vice versa. Free sinusoidal oscillations can occur during the mutual transformation of any physical types of energy.This circumstance is the motive to make an oscillator, in which free sinusoidal oscillations are accompanied by the transformation of the kinetic energy of an inert element into the kinetic energy of another inert element. There are no elements with a different reactivity character in such an oscillator. This type of an oscillator is essentially monoreactive.