Approximation

Purity is at best an approximation.

It's not the exact value, but rather an approximation.

In computer science, a method of investigation based on the progressive approximation of a problem is called a heuristic.

It's only an approximation.

The value is only an approximation of the actual function value.

At the back of the cupboard, you find your interview skirt, the one you bought when you had cash to purchase an approximation of the fashion spreads you mulled over in magazines.

For some loanwords borrowed from French into English, the standard English pronunciation is an approximation of the French, as with denoument.