A theoretical measure of the execution of an algorithm, usually the time or memory needed, given the problem size n, which is usually the number of items. Informally, saying some equation f(n) = Θ (g(n)) means it is within a constant multiple of g(n). The equation is read, "f of n is theta g of n". Formal Definition: f(n) = Θ (g(n)) means there are positive constants c₁, c₂, and k, such that 0 ≤ c₁g(n) ≤ f(n) ≤ c₂g(n) for all n ≥ k. The values of c₁, c₂, and k must be fixed for the function f and must not depend on n.