Lambda

Lambda (uppercase/lowercase Λ λ) is a letter of the Greek alphabet. It is used to represent the "l" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 30. Letters that came from it include the Roman L and Cyrillic Л. 
In mathematics, the lowercase λ is used as a variable for eigenvalues in linear algebra, and as the parameter of both exponential distribution and Poisson distribution. In physics, the same symbol is used as a variable representing wavelength.