The constant of variation is a fixed number that relates two variables which are directly or inversely proportional. It is represented by the symbol $k$ in equations like $y = kx$ (direct variation) and $xy = k$ (inverse variation).
Direct Variation: A relationship where one variable is equal to a constant times another variable ($y = kx$).
Inverse Variation: A relationship where one variable multiplied by another equals a constant ($xy = k$).
Proportionality Constant: Another term for the constant of variation; it represents the factor that relates two proportional quantities.