Beta

Beta (uppercase/lowercase Β β), is a letter of the Greek alphabet, used to represent the "b" sound in Ancient Greek and "v" in Modern Greek. In the system of Greek numerals, it has a value of 2. Letters that came from it include the Roman B and Cyrillic Б.
The sounds [b] and [v] resemble each other a lot. In Ancient Greek the "b" sound became "v" around 200 BC. Therefore, the letter B means the "v" sound in the Cyrillic alphabet, and a new letter, Б, was made for [b]. 
The letter beta is also used in finance, science and mathematics. For example. the beta function is named after the capital letter Β, and the probability of type II error in statistics is written with the lowercase β.