Its a very simple concept and so there's no reason to give it a scientific name which doesnt mean anything to most people

And by the way I would say that entropy isnt even a useful term here. Entropy is defined as

k ln(O)

where O is the total number of microstates (here the number of unique keys that can be made in the system) and k is the boltzmann constant. For the purpose of brute-force attacks we're interested in the total number of microstates O, and so the logarithmic scaling of entropy isn't meaningful.