Approximation made to reduce the computation/complexity of math required for a given simulation. Recently, thanks to cheap supercomputers, higher-order approximations can be used to design things that were previously impossible to simulate.

Sometimes, the model reflects the limits of understanding--for instance light as a wave and quantum mechanics.

bees can't fly, according to standard fluid dynamics.

In the Milan polytechnic (an engineering and architecture-centered public university) they say that for civil engineers infinity equals 5.

They also say that for architects it equals duh, but that is just standard inter-species hatred.