The expression "2πk" represents a mathematical concept often used in trigonometry and periodic functions. Here, "π" (pi) is a constant approximately equal to 3.14, and "k" is an integer. This formula is significant because it describes the angles in radians that correspond to full rotations around a circle, where each complete rotation is equal to 2π radians. In practical terms, "2πk" is used to express the periodic nature of functions like sine and cosine. For example, if you have a wave that repeats every full rotation, you can use "2πk" to find the angle at which the wave returns to its starting point, making it essential in fields like physics and engineering.