We describe the thermodynamic state of a highly confined single-phase and single-component fluid in a slit pore using Hill's thermodynamics of small systems. This theory was more recently named nanothermodynamics. We start by constructing an ensemble of slit pores for controlled temperature, volume, surface area, and chemical potential. We present the integral and differential properties according to Hill, and use them to define the disjoining pressure. We identify all thermodynamic pressures by their mechanical counterparts in a consistent manner, and investigate the identification by molecular dynamics simulations. We define and compute the disjoining pressure, and show that it contains the standard definition. We compute the entropy and energy densities, and find in agreement with the literature, that the forces at the wall are of an energetic, not entropic nature. The subdivision potential is zero for this slit pore with large walls, but unequal to zero for related sets of control variables. We show how Hill's method can be used to find new Maxwell relations of a confined fluid, in addition to a scaling relation, which applies when the walls are separated far enough. By this expansion of nanothermodynamics, we set the stage for further developments of the thermodynamics of confined fluids, a field that is central in nanotechnology.