The word Piezoelectricity comes from Greek and means “electricity by pressure” (Piezo means pressure in Greek). This name was proposed by Hankel  in 1881 to name the phenomenon discovered a year before by the Pierre and Jacques Curie brothers . They observed that positive and negative charges appeared on several parts of the crystal surfaces when comprising the crystal in different directions, previously analysed according to its symmetry.
Figure 1a shows a simple molecular model; it explains the generating of an electric charge as the result of a force exerted on the material. Before subjecting the material to some external stress, the gravity centres of the negative and positive charges of each molecule coincide. Therefore, the external effects of the negative and positive charges are reciprocally cancelled. As a result, an electrically neutral molecule appears. When exerting some pressure on the material, its internal reticular structure can be deformed, causing the separation of the positive and negative gravity centres of the molecules and generating little dipoles (Fig. 1 b). The facing poles inside the material are mutually cancelled and a distribution of a linked charge appears in the material’s surfaces (Fig. 1c). That is to say, the material is polarized. This polarization generates an electric field and can be used to transform the mechanical energy used in the material’s deformation into electrical energy.
Figure 2a shows the piezoelectric material on which a pressure is applied. Two metal plates used as electrodes are deposited on the surfaces where the linked charges of opposite sign appear. Let us suppose that those electrodes are externally short circuited through a wire to which a galvanometer has been connected. When exerting some pressure on the piezoelectric material, a linked charge density appears on the surfaces of the crystal in contact with the electrodes. This polarization generates an electric field which causes the flow of the free charges existing in the conductor. Depending on their sign, the free charges will move towards the ends where the linked charge generated by the crystal’s polarization is of opposite sign. This flow of free charges will remain until the free charge neutralizes the polarization effect (Fig. 2a). When the pressure on the crystal stops, the polarization will disappear, and the flow of free charges will be reversed, coming back to the initial standstill condition (Fig. 2b). This process would be displayed in the galvanometer, which would have marked two opposite sign current peaks. If a resistance is connected instead of a short-circuiting, and a variable pressure is applied, a current would flow through the resistance, and the mechanical energy would be transformed into electrical energy.
The Curie brothers verified, the year after their discovery, the existence of the reverse process, predicted by Lippmann (1881) . That is, if one arbitrarily names direct piezoelectric effect, to the generation of an electric charge, and hence of an electric field, in certain materials and under certain laws due to a stress, there would also exist a reverse piezoelectric effect by which the application of an electric field, under similar circumstances, would cause deformation in those materials.
In this sense, a mechanical deformation would be produced in a piezoelectric material when a voltage is applied between the electrodes of the piezoelectric material, as shown in Fig.2. This strain could be used, for example, to displace a coupled mechanical load, transforming the electrical energy into mechanical energy.