Nexus

noun

1. A form of connection.
2. A connected group.
3. The centre of something.
4. In Ancient Rome, a person who had contracted a nexum or obligation of such a kind that, if he failed to pay, his creditor could compel him to work as a servant until the debt was paid.
5. The relationship between a vendor and a jurisdiction for the purpose of taxation, established for example by the vendor operating a physical store in that jurisdiction.