Final answer:
A mutation that changes an amino acid coding codon to a stop codon is known as a nonsense mutation, typically resulting in a truncated and likely nonfunctional protein.
Step-by-step explanation:
A mutation that changes an amino acid coding codon to a stop codon is classified as a nonsense mutation. In such a mutation, the normal codon that would code for an amino acid is replaced with one of the three stop codons (UGA, UAA, or UAG), which signals the end of the protein-coding sequence. The resulting protein is usually truncated and often nonfunctional because the translation process terminates prematurely. This can have serious implications for the organism because the shorter protein might be unable to fulfill its normal function within the cell.