Quiescent point is also known as the operating point, equilibrium point ... and maybe some other names.
It is the constant value about which the circuit is operating. The idea goes beyond diodes and transistors.
It can apply to any device with nonlinear characteristics ... in these types of devices, the incremental slope
depends heavily on the constant values about which the operation takes place.
Take a look at the neon lamp characteristics here https://en.wikipedia.org/wiki/File:Linear_voltage-driven_1000.jpg
Depending on where you are on the "S" shaped curve you can have a positive or negative slope and therefore
behavior could be a positive or negative incremental resistance.