A government by one man is called -

A government by one man is called Autocracy.

• Autocracy
- English Meaning: a government in which one person possesses unlimited power.
- বাংলা অর্থ: একনায়কতন্ত্র।

• Other options:
- A government by the wealthy - Plutocracy (ধনিকগোষ্ঠী),
- A government by a king or a queen - Monarchy (রাজতন্ত্র),
- A body of nonelected government officials - Bureaucracy (আমলাতন্ত্র)।

Source: 1. Merriam-Webster Dictionary.
2. Accessible Dictionary.