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Counter Mode (CTR) is an encryption mode, that builds a random-access stream-cipher from a block-cipher.

Counter Mode (CTR), also known as Segmented Integer Counter (SIC) or Integer Counter Mode (ICM), makes a block-cipher into a random-access stream cipher by generating a keystream using the block-cipher that is XORed with the plaintext to produce the ciphertext. CTR mode generates keystream blocks by encrypting successive values of a non-repeating counter with the block-cipher.

See the wikipedia page for more information.