$\theta$ — Theta

Lowercase theta (θ) almost always denotes an angle — a launch angle, an angle of incidence, an angular position. It is the default symbol whenever direction is measured from a reference line.

SI unit: rad (or degrees in practice)

Used in

• Projectile launch angle
• Snell's law n₁sinθ₁ = n₂sinθ₂
• Angular position in circular motion