Two-thirds of something is an amount that is two out of three equal parts of it. Two-thirds of householders in this country live in a mortgaged home Two-thirds is also a pronoun. The United States and Russia hope to conclude a treaty to cut their nuclear arsenals by two-thirds. Two-thirds is also an adverb. Do not fill the container more than two-thirds full A second book has already been commissioned and is two-thirds finished. Two-thirds is also an adjective. the two thirds majority in parliament needed to make constitutional changes