The usual definition of a contact closer/interrupter is the device's at rest state, that is, no external influence. A NC relay, for example, opens when voltage is applied. A NO momentary switch requires moving a lever or pushing a button to complete the circuit. Likewise, a magnetic reed/contact switch changes it's at rest state when under the influence of a magnet.
NO = proximity to a magnetic field closes the contacts
NC = proximity to a magnetic field opens the contacts
That's also the same way the I/O Linc contacts are labeled.