Opaque
An opaque material blocks light completely by absorbing or reflecting it, so nothing can be seen through it, like wood, metal, or brick.
Definition
An opaque material does not let any light through: wood, metal, brick, and a hand are all opaque, which is why they cast shadows, and opaque things still interact with light by absorbing or reflecting it, so an opaque object can be any color, black, or shiny. It transmits no light, with everything striking it either absorbed or reflected, casting sharp shadows, its color coming from the wavelengths it reflects, and opacity depends on both thickness and the type of wave: aluminum foil is opaque to visible light while thin paper may be translucent, and skin is opaque to visible light yet transparent to X-rays. Opacity means the transmitted fraction $T = (1 - R)^2 e^{-\alpha x}$ is effectively zero, either because the absorption coefficient $\alpha$ is large, as when photon energy matches electronic or vibrational transitions or free electrons in metals absorb across the visible, or because the material is thick enough that $\alpha x \gg 1$; in metals, fields penetrate only to the skin depth $\delta = \sqrt{2/(\mu\sigma\omega)}$, a few nanometers for visible light, so even thin foils are opaque and highly reflective, and astronomers describe opacity with optical depth $\tau = \alpha x$, a medium being opaque when $\tau \gg 1$, which is why the Sun has a visible surface even though it is gas all the way down.
Example
Holding a book in front of a flashlight darkens the wall behind it, since the book blocked all the light; a brick wall reflects about $20$ to $30\%$ of sunlight, absorbs the rest, and transmits none, so its shaded side is dim rather than perfectly black because the only light there arrives indirectly from the sky. Copper at visible frequencies has a skin depth of roughly $3\ \text{nm}$, so a $30\ \text{nm}$ film, ten skin depths thick, transmits about $e^{-10} \approx 4.5 \times 10^{-5}$ of the light, and the Sun's photosphere is the layer where $\tau \approx 2/3$ looking inward, beyond which light from deeper layers cannot escape, giving the star its sharp-edged look.
Key Insight
Whether something is opaque depends on the wave: a wooden door blocks light but lets sound through, a glass window lets light through but blocks most sound, and radio waves pass through walls that stop both. Nothing is opaque to every wave; neutrinos pass through a light-year of lead with only a $50\%$ chance of stopping, and X-rays see bones through flesh, so opaque always needs the question, opaque to what.