A common supersequence of strings $s$ and $t$ is said to be a shortest common supersequence of $s$ and $t$ if there does not exist a shorter common supersequence. That is, a shortest common supersequence is a common supersequence of minimum length.

For example, although you may verify that although "GACCTAGGAACTC" acts as a common supersequence of "ACGTC" and "ATAT", it is not a shortest common supersequence of the two strings; however, "ACGTATC" does serve as a shortest common supersequence of "ACGTC" and "ATAT".