A boat is generally qualified as a yacht when it’s at least 33ft or 10m in length, has a luxury appearance and judged to have good aesthetic features. A boat is classified (IMO) as a yacht from 79ft or over 24m. Either boats powered by engine or propelled entirely by sail can be typified as a yacht.
A yacht is qualified as a superyacht when it’s at least over 131ft or 40m in length.
A superyacht is qualified as a megayacht when it’s at least over 328ft or 100m in length.