Homonuclear correlation spectroscopy or COSY is a 2D NMR technique that identifies coupled protons.

Consider the COSY spectrum of ethyl acetate. The corresponding 1D proton NMR spectrum is plotted along the horizontal and vertical axes.

The three spots on the diagonal correspond to the three peaks in the 1D spectrum, making COSY spectra symmetric about the diagonal.

The signals above and below the diagonal are called cross-peaks or off-diagonal peaks.

The off-diagonal peaks correlating the multiplets at 1.25 and 4.2 ppm indicate that the methyl protons are coupled to the methylene protons.

The signal at 2 ppm is not correlated to any cross-peaks because the protons are not involved in homonuclear coupling.

So, COSY spectra can identify coupling networks and therefore structural connectivity in molecules with complicated 1D spectra.