Yotta- is an SI prefix meaning one septillion. The name is a modification of Greek "okto" meaning eight. It is the largest official SI prefix, with no SI prefixes for numbers past a septillion. For extensions to the SI prefixes other people have made, see the articles on extended SI prefixes starting with Prefix 10^27.
The reciprocal of this prefix is yocto-. The name is a modification of Latin "octo" meaning eight. It is the smallest official SI prefix.