clearance

noun

1. an act or instance of clearing
2. the clear space or distance between moving objects or mechanical parts, or between a moving object and that which it passes through, over, under, etc.
3. official, esp. governmental, authorization allowing a person to examine classified documents, participate in confidential projects, etc.
4. 1. a certificate from the collector of customs authorizing a ship to enter or leave port
      also clearance papers
   2. the act or process of meeting the requirements for getting this certificate
5. Aeron. permission, usually from a control tower, to take off, land, etc.
6. Banking the adjustment of debits and credits, exchange of checks, etc. in a clearinghouse