Block cipher is an encryption method which the encryption algorithm operates on a plaintext block of n bits and produces a block of n bits ciphertext.
Block Cipher has \({2}^{n}\) possible different plaintext block to encrypt.
Nonsingular Transformation Nonsingular Transformation mean the encryption algorithm must be reversible (Nonsingular) to decrypt the ciphertext into the original plaintext.
The ciphertext must be unique for each plaintext block.
Example: Encryption block cipher with a plaintext block length n=4 bits and ciphertext, with \({2}^{4}=16\) possible input states

AES takes a plaintext block 16 bytes (128 bits) and key length 16, 24, 32 bytes (128, 192, 256 bits) AES-128, AES-192, AES-256 depending on key length.
The input to encryption or decryption algorithm in AES is 128 bits block = 16 bytes as 4x4 matrix, this matrix is copied into a state array, then the state array passes through and modified in each stage and then the state array is copied to output matrix.