BER, or Bit Error Rate, is a measure used in telecommunications to quantify the number of errors in a transmitted data stream. It is defined as the ratio of the number of erroneous bits to the total number of bits transmitted. A lower BER indicates a more reliable communication channel, while a higher BER suggests more errors and potential issues in data transmission. BER is crucial for assessing the performance of various communication systems, including wireless networks, satellite communications, and fiber optic links. Engineers use this metric to optimize systems and ensure data integrity, making it essential for maintaining quality in digital communications.