axⁿ + bxⁿ-¹ + cxⁿ-² and so on are used to represent coefficients of polynomials sometimes.
a is the symbol for acceleration
B is the symbol for magnetic field
c is the speed of light, as in e = mc². c approximates to 3 × 10⁸ m/s
dy/dx in calculus
e is Euler's number and approximates to 2.71828
f(x) and f'(x) in calculus
It's customary to denote centroids by G.
h is Planck's constant
i² = -1. I is also the symbol for electric current
j is used in electrical engineering to denote the imaginary components of complex numbers
k is used to represent constants, like the constant of proportionality and the spring constant in the formula for elastic potential energy.
l is used in geometry to denote lines. L is used to represent the Lagrangian in Lagrangian mechanics.
m is the slope of a line, as in y = mx + c
O is the origin (0,0) on a Cartesian plane.
p(x) is a standard notation for polynomials.
Q (blackboard bold/boldface) is used to represent rational numbers. Q or q is the symbol for charge.
R (blackboard bold or boldface) is used to represent real numbers.
s is the symbol for distance in physics. S is used to denote sample space.
t is used to represent time in applied math. T(x) is also a transformation function that defines how x will be transformed.
v is used widely in physics. In kinematics, it represents velocity or final velocity. In optics, it is the distance between the image and the mirror or lens. V is also potential difference or voltage.
W is used to denote whole numbers.
As Johnny101 pointed out, all letters can be used as variables. All letters can also be used to name points in geometry.
