They are not interchangeable, though both can appear in the same photo.

Diffraction spikes are the star-like rays extending from bright point light sources. They are caused by light diffracting at the edges of the lens aperture. The effect depends on the aperture shape and f-stop: stopping down usually makes spikes more visible. Blade count matters too: with an even number of aperture blades, the visible spike count is usually the same as the blade count; with an odd number, it’s usually double.

Lens flare is caused by unwanted reflections/scattering inside the lens. It often appears as ghost shapes, colored blobs, or a washed-out haze (veiling glare), especially when bright light hits the lens at certain angles. Lens hood use, lens design, coatings, and element count can affect it.

So: diffraction spikes = diffraction at the aperture; lens flare = internal reflections/scatter in the lens. A single image can show both at once, but for different reasons.