Resolving power
Definition
Resolving power is an instrument's ability to see two close objects as separate instead of one blurry blob — the reason bigger telescopes see finer detail.
Technical definition
The Rayleigh criterion: two point sources are just resolved when the central maximum of one diffraction pattern falls on the first minimum of the other, θ ≈ 1.22λ/D, where D is the aperture diameter.
Example
A 10 cm telescope at 550 nm light resolves about 1.22×550×10⁻⁹/0.1 ≈ 6.7 microradians — roughly a 1 m crater on the Moon from Earth.
Common confusion
Magnification does not improve resolving power. A cheap telescope at 500× just makes a blurry blob bigger; only a larger aperture (or shorter wavelength) reveals genuinely finer detail.