being used as a fairlead
is a block
, padeye, ring or any other device which controls the path of a line on a boat.
When a line is not running correctly where it is supposed to go, or is snagged or stuck, it is said to be "fouled". When a line is running correctly and is not fouled, it is said to be "fair".
A fairlead is simply a device that "leads" a line "fair".
A padeye can be used as a fairlead, but lines should only be run through padeyes at shallow angles. For sharp angles (especially when heavy loads are required), a block should be used.