The type of government in which one person holds supreme power

Autocracy. It comes from Autokrator, which in Greek means "self-rule" and "one who rules by himself". Modern forms of autocracy are absolute monarchies (such as Swaziland) and dictatorships (such as North Korea)

it can either be a Monarchy - a government in which the supreme power is lodged in the hands of a monarch who reigns over a state or territory, usually for life and by hereditary right; the monarch may be either a sole absolute ruler or a sovereign - such as a king, queen or prince - with constitutionally limited authority.

or a dictatorship were one person rules the whole place and that one person holds all supreme power