A geometric progression (or geometric sequence) is a sequence of numbers where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio. For example, in the sequence 2, 6, 18, 54, each term is multiplied by 3 to get the next term. Geometric progressions can be expressed in the form a, ar, ar^2, ar^3, \ldots , where a is the first term and r is the common ratio. They are commonly used in various fields, including finance, biology, and computer science, to model exponential growth or decay.