Assuming I hold a single entry Cyprus visa, can I use it to enter Romania if the single entry is valid and unused?
Both short-term and long-stay visas issued by Cyprus are valid in Romania for transit or visits up to ninety days, provided any entry limits have not been exhausted.
Quoting from the Romanian Department of Foreign Affairs web site:
Holders of Schengen visas with two or multiple entries, national visas or residence permits issued by Schengen Member States. Starting July 11, 2014, in keeping with the provisions of Decision No 565/2014/EU, Romania acknowledges as being equivalent to its own national visas, for transit and for stays up to 90 days per period of 180 days the following:
The two or multiple entry short-stay visa issued by a Schengen member state;
The limited territorial validity visa issued by a Schengen member state;
The short-stay visa issued by Bulgaria, Cyprus or Croatia;
The long-stay visa issued by a Schengen member state;
The long-stay visa issued by Bulgaria, Cyprus or Croatia;
The residence permit issued by a Schengen member state;
The residence permit issued by Bulgaria, Cyprus or Croatia
The long-term resident's EC residence permit (the validity of which is of 5 years or more) issued by a member state of the European Union.
Attention! The number of entries as well as the right of stay established as per the Schengen visas must not have been exhausted. The right of stay on the territory of Romania shall not exceed the right of stay granted as per the visas/residence permits issued by the Schengen Member States