Α - alpha - capital First letter of the Greek alphabet, therefore takes on the meaning of first, best, primary, number one, etc. Used as an adjective as in alpha male or alpha version.
α - alpha -small 1. Mathematics. Direct proportionality. 2. Physics. Angular acceleration. Alpha particle and alpha decay.

Β - beta - capital Second letter of the Greek alphabet, therefore takes on the meaning of being next to first, or almost there, as in beta testing.
β - beta - small 1. Phonology. Symbol for the voiced bilabial fricative [IPA]. 2. Special Relativity. Symbol for the speed of an object relative to the speed of light. 3. Particle Physics. The beta particle and beta decay.

Γ - gamma - capital 1. Electrical Engineering. Symbol for the reflection coefficient. 2. Mathematics. The gamma function, as related to factorials. 3. Probability & Statistics. The gamma distribution.
γ - gamma - small 1. Physics. Symbol for photon. Also symbol for adiabatic index of a gas. 2. Astronomy. Gamma factor and gamma rays. 3. Mathematics. Symbol for the Euler-Mascheroni constant (sometimes shortened to Euler Constant).

Δ - delta - capital 1. Data Converters. Delta-Sigma conversion (Δ-process. A favorite audio data converter technology using a delta change as the heart of the process. 2. Geography. Name for the triangular shape caused by sediment deposits at the mouth of a river. 3. Physics. Delta particles in particle physics.
δ - delta - small 1. Calculus. Symbol used to represent a very small change in a value, often called a delta change; a finite increment in a variable. 2. Mathematics. Symbol for the Dirac delta function (or just delta function) ; symbol for the Kronecker delta. 3. Proofreading. Symbol for deletion.

Ε - epsilon - capital Rarely seen since it is too close to the English letter "E."
ε - epsilon - small 1. Electrical Engineering. Symbol for the permittivity constant. 2. Mathematics. A very small quantity; in tensor calculus, the Levi-Civita symbol, also called the permutation symbol; set membership. 3. Computer Science. Symbol for an empty string.

Ζ - zeta - capital Rarely seen since it is too close to the English letter "Z."
ζ - zeta - small Mathematics. The Riemann zeta function and other zeta functions.

Η - eta - capital Rarely seen since it is too close to the English letter "H."
η - eta - small 1. Thermodynamics. Symbol for the efficiency of a Carnot heat engine. 2. Physics. Symbol for refractive index of an optical medium.

Θ - theta - capital Symbol for death since it is an abbreviation for the Greek word thanatos, meaning the personification of death.
θ - theta - small 1. Trigonometry. Symbol for an unknown angle. 2. Geometry. Symbol for a plane angle. 3. Phonology. Symbol for voiceless dental fricative [IPA].

Ι - iota - capital Rarely seen since it is too close to the English letter "I."
ι - iota - small The smallest letter in the Greek alphabet, hence the popular English usage to mean small, as in, "It doesn't matter one iota."

Κ - kappa - capital Rarely seen since it is too close to the English letter "K."
κ - kappa - small 1. Physics. Alternate symbol for adiabatic index of a gas. 2. Mathematics. The Kappa Curve.

Λ - lambda - capital 1. Particle Physics. Symbol for lambda particles.
λ - lambda - small 1. Electromagnetic Waves. Symbol for wavelength. 2. Mathematics. Symbol for eigen values in linear algebra.

Μ - mu - capital Rarely seen since it is too close to the English letter "M."
μ - mu - small 1. Symbol for micro- and micron. 2. Statistics. Symbol for the mean. 3. Physics. Symbol for coefficient of friction. 4. Electromagnetism. Symbol for permeability.

Ν - nu - capital Rarely seen since it is too close to the English letter "N."
ν - nu - small 1. Physics. Symbol for the frequency of a wave. 2. Particle Physics. Symbol for any of the three kinds of neutrinos.

Ξ - xi - capital 1. Particle Physics. Symbol for cascade particles.
ξ - xi - small 1. Mathematics. Symbol for the Xi-Function.

Ο - omicron - capital Rarely seen since it is too close to the English letter "O."
ο - omicron - small Ditto to above.

Π - pi - capital 1. Mathematics. Symbol for product, as in multiplication.
π - pi - small 1. Symbol for the ratio of the circumference of a circle to its diameter; a transcendental number, equal to approximately 3.14159. Note that the ratio of the circumference of an igloo to its diameter equals Eskimo Pi.

Ρ - rho - capital Rarely seen since it is too close to the English letter "P."
ρ - rho - small 1. Physics. Symbol for resistivity and density.

Σ - sigma - capital 1. Mathematics. Symbol for summation. 2. Data Converters. Delta-Sigma conversion (Δ-Σ) process. A favorite audio data converter technology using delta change and summation.
σ - sigma - small 1. Statistics. Symbol for the standard deviation.

Τ - tau - capital Rarely seen since it is too close to the English letter "T."
τ - tau - small 1. Mechanics. Symbol for torque. 2. Electronics. Symbol for the time-constant in an RC circuit.

Υ - upsilon - capital Rarely seen since it is too close to the English letter "Y."
υ - upsilon - small Rarely seen since it is too close to the English letter "u."

Φ - phi - capital 1. Physics. Symbol for magnetic flux.
φ - phi - small 1. Electronics. Symbol for phase or phase shift. 2. Mathematics. Symbol for the number 1.618 ... the golden number. See Phi.

Χ - chi - capital Rarely seen since it is too close to the English letter "X."
χ - chi - small 1. Physics. Symbol for electric susceptibility. 2. Phonology. Symbol for the voiceless uvular fricative [IPA].

Ψ - psi - capital Rarely seen since it is difficult to distinguish between the capital and the small psi symbols.
ψ - psi - small 1. Quantum Mechanics. Symbol for the wave function. Symbol for psi-network used in consumer audio.

Ω - omega - capital 1. Electronics. Symbol for ohm. 2. Last letter of the Greek alphabet so takes on the meaning of being last.
ω - omega - small 1. Electronics. Symbol for angular velocity, i.e., frequency in radians/second = 2πf.