Semiconductor & Computer Engineering
Not Connected Pin (NC pin) or Not Connected Terminals are pins found on various integrated circuit packages. Those pins have no functional purpose to the outside circuit (but may have unexposed functionality). Those pins should not be connected to any of the circuit connections - i.e. they should be left floating.
When deciding how to proceed with NC pins, consult the accompanying datasheet for explicit NC pin recommendations. While most manufacturers simply state that such pins should be left floating, in other cases they may have more specific recommendations such as connected to the ground rail for some additional, but unnecessary benefits.
IC chips are mostly mass produced in a limited array of packages. Most designers are thus restricted to one of those pre-defined packages. Sometimes the device that's designed does not utilize all the pins for various reasons. It may also be as a result of post-package trimming. Some manufacturers may also designate some pins for their internal use only (e.g. calibration or testing). When that happens, those pins gets labeled "NC" or "Not Connected".
Use in EDA
When designing circuits (e.g. for PCBs), marking NC pins correctly is very important because many of those tools with Electrical Rule Check capabilities can detect when something is mistakenly connected to them thereby issuing a warning.