Transit method; transit depth; impact parameter কী?

The transit method detects a planet by the periodic dimming of its star as the planet crosses the stellar disc. The transit depth $\delta$ is the fractional drop of the star’s flux during the transit. The impact parameter $b = a\cos i/R_\star$ is the smallest distance between the centres of the planet and the star, as seen on the sky, in units of the stellar radius $R_\star$, for an orbit of radius $a$ and inclination $i$.