A mutation involving a single base of DNA. If this occurs at a silent site, it will have no effect on the amino acid sequence. Alternatively, it may cause an amino acid to be replaced with another one - this may alter the function or effectiveness of the resulting protein. It may also turn a normal codon into a stop codon (or vice-versa), thereby altering the length of the protein.

Other types of point mutations are point insertions and point deletions. A single DNA base gets left out of the strand for one reason or another in the case of a deletion, while in an insertion an extra base gets stuck in. This can severely screw up all subsequent codes, leading to a lot of amino acids that aren't what they should be. Think touch typing with your hands moved off of the home row. Gibberish.