A routing number is a 9-digit bank code which is identifies a financial institution and they Federal Reserve Region they belong to. Synonyms for a routing number is 'routing transit number' or 'ABA number'.
The ABA Routing Number was developed by the American Bankers Association (ABA) in 1910.
The routing number in combination with an account number is used for electronic deposits, a.k.a. wiring money to a bank account or to withdraw money from bank accounts electronically.
If you need to know the routing number for your U.S. bank account then you can find it e.g.
on your checks printed at the bottom line: it is the left hand side 9-digit number.
You can also call or visit your bank. Some banks also list the routing number for your account when you go online, unfortunately this is not common practice yet.
Federal Reserve: You can search for routing numbers or download a list on the website of the Federal Reserve. They maintain a text file list which is updated regularly. Find the weblink further below.
American Banker Assoc. (ABA): Their website provides further insight into the subject.
Accuity: It is the Official ABA Registrar of Routing Numbers for ABA and responsible for the assignment of routing numbers in accordance with the Routing Number Administrative Board Routing Number Policy. Accuity is ABA's Official Registrar since 1911.