Transparent
A transparent material lets light pass straight through so objects behind it can be seen clearly, such as clear glass, water, or air.
Definition
A transparent material lets light go straight through so what is on the other side can be seen clearly, such as window glass, clean water, and air, though transparent does not mean invisible: a small amount of light reflects off a window's surface and bends slightly, which is why the glass itself is still visible. It transmits light with little scattering or absorption, so images pass through clearly even though light slows down and refracts inside the material and a small fraction reflects at each surface, and transparency depends on wavelength; ordinary glass is transparent to visible light but blocks most ultraviolet. A material is transparent at a given wavelength when its photons lack the energy to excite electronic transitions (glass has a band gap around $9\ \text{eV}$ against $3\ \text{eV}$ visible photons), when there are no matching vibrational modes, and when the structure is homogeneous on the scale of the wavelength so scattering is negligible, with transmission following $T = (1 - R)^2 e^{-\alpha x}$ for a slab; metals are opaque because free electrons absorb and re-radiate light, while ultra-pure silica fiber is so transparent at $1550\ \text{nm}$ that light loses only about $0.2\ \text{dB/km}$.
Example
A book can be read through a clear plastic sheet, and fish can be watched through the glass of an aquarium; ordinary window glass transmits about $90\%$ of visible light, reflects about $8\%$, and absorbs the remaining $2\%$, with a thick block of glass looking slightly green because it absorbs a bit more red light than green. Silica fiber with $0.2\ \text{dB/km}$ loss passes $10^{-0.02} \approx 95.5\%$ of light per kilometer, and after $50\ \text{km}$ that is still $10\%$, detectable, which is why undersea cables need repeaters only every few dozen kilometers, while window glass at about $30\ \text{dB/km}$ would be effectively black after a few hundred meters.
Key Insight
Water is transparent in a glass, yet the ocean looks blue and a cloud looks white: depth adds absorption and droplets add scattering, so the same substance can be transparent, colored, or opaque depending on how it is arranged. Transparency is really a statement about energy levels rather than about having holes for light to pass through: diamond and glass are transparent because their electrons have no allowed state to jump to when a visible photon arrives, so the photon passes untouched.