Which is it?
Entities that receive federal assistance (i.e. federal funds, federal grants, and federal awards) are subject to audits in order to ensure that the federal assistance programs are utilized in compliance with the federal government. Before 1984 these audits were performed per program rather than per entity. In other words, entities with multiple assistance sources were subject to multiple audits and the costs of those audits.
For the sake of efficiency and cost-effectiveness the Single Audit Act of 1984 was passed. The act earned its name because its purpose is to consolidate these audits, allowing entities to receive one audit over all of their federal assistance.
Due to this act these audits are referred to as "Single Audits".