Countries where regular elections are held with a single party/major candidate. Minor opposition candidates may be present but may not capture a large share of the votes. A positive indicator for a democratic system is regular transition of power, and a system in which a single person is consistently re-elected, or a single party. If the voter has only 1 choice in an election, the election is less democratic than one with multiple choices
This means countries like Japan or Singapore, where while technically multi-party, in practice the opposition parties never win. Obviously a country where other parties are outlawed (e.g. communist countries) is not a single party democracy, it's just a single party dictatorship.