The paper observes the concept of “potential vorticity in the ocean”, including the history of the appearance of this term. Various aspects and basic formulas are discussed, which are used in practice to calculate potential vorticity, and the conditions for the fulfilment of its conservation law are considered. Two approaches to potential vorticity are considered: by the Ertel and Rossby formulas. To illustrate the main conclusions, we present estimates of the potential vorticity for the quasi-permanent anticyclonic Lofoten vortex in the Norwegian Sea. We use the data of the global oceanic reanalysis GLORYS12V1 for calculations of the potential vorticity. We establish that the potential vorticity by the Ertel is a kinematic characteristic determined by the vortex core, in which the typical values are zero, while they can reach values of –10 × 10–10 m–1 s–1 at the core-periphery, where the concentration of the isopycnals is observed. The vertical and horizontal sections of the relative and potential vorticities are also constructed. It is found that the main contribution in the potential vorticity are the horizontal components while vertical components are significantly less. In contrast to the potential vorticity by Ertel, the potential vorticity by Rossby in the quasi-geostrophic approximation is the dynamic characteristic. The main contribution is made by the relative vorticity, which characterizes the rotation of the particles. The volumetric potential vorticity characterizes the vortex power. The highest potential vorticity value corresponds to the horizon of 500 m and is –1.3 × 10–5 s–1; at horizons below 1000 m, the values do not exceed 1.0 × 10–5 s–1, and at a horizon of 3000 m it is about 0.5 × 10–5 s–1. The volumetric potential vorticity for the Lofoten vortex is –9.82 × 106 s–1. For the core (up to 1000 m), it is –2.28 × 108 s–1.