"Irreducible" refers to something that cannot be simplified or broken down into smaller parts. In mathematics, an irreducible fraction is one where the numerator and denominator have no common factors other than 1. This means it is in its simplest form and cannot be reduced further. In other contexts, such as science or philosophy, an irreducible concept or entity cannot be explained or represented by simpler components. For example, in biology, certain structures or systems are considered irreducible if removing any part would cause them to lose their essential function.