The unmarried certificate is a single status proof. This is legal proof that you have been single and have no dependents from any matrimony. However, a divorcee and a widowed can also be an unmarried person and hence, are applicable for it.
This document can also be called a single status certificate, or affidavit, nulla osta, certificate of no impediment, or any other name. You need to visit the civil authority where you come from. It will issue it, which is later attested by the state and foreign ministry.
These attestation services are all payable. However, it’s really inexpensive but full of hassles. So, it’s better to hire a consultant or legal matter expert for getting it quickly.