When we want to mean a government by the richest class we use the term ____.

Plutocracy (noun):
Meaning: Government by the wealthy.
Bengali meaning: ধনিকগোষ্ঠী; ধনিকতন্ত্র।

The English & Bengali meaning of the following words:

Olicracy -
Meaning: A small group of people having control of a country, organization, or institution.
Bengali meaning:সর্বময় ক্ষমতার অধিকারী ক্ষুদ্র এক গোষ্ঠীর শাসন; গোষ্ঠীশাসন; গোষ্ঠীশাসিত দেশ; এরূপ গোষ্ঠী।
Cryptocracy -
Meaning: A form of government where the real leaders are hidden, or merely unknown.
Aristocracy -
Meaning: The highest class in certain societies, especially those holding hereditary titles or offices.
Bengali Meaning:অভিজাততন্ত্র।

Source: Oxford Dictionary & Bangla Academy Dictionary.