Bill Broad gets all scared about nonlinearity today. Ever since the word came into its current popularity, I haven't been able to understand the mystification that sprang up around it.
If I recall correctly, Los Alamos had a Center for Nonlinear Studies back in the 1980s that eventually contributed some members to the Santa Fe Institute, where nonlinearity is part of what they do, although I have to say that I haven't been hearing the mystification from them lately, since they do know what they're doing.
Back in the eighties, I wondered why they chose that word, but I was working in a different enough field that I didn't have a chance to ask the question. Any of us who have had algebra have encountered nonlinearity: a quadratic equation is not linear. Or perhaps it is, if you include any smooth mathematical function as linear.
So perhaps nonlinearity refers to discontinuities, although I don't think Bill Broad knows that. He just knows that nonlinear is a scary word, that thing about butterflies in Brazil and typhoons in the Pacific. But I would think that nonlinear also could mean an effect that just goes to zero pretty quickly, although that's less interesting and scary.
Perhaps Broad means unpredictable, but what he's talking about, uranium enrichment, has a set of equations that describes it adequately. He just means that it gets easier to enrich uranium as the enrichment gets higher.
But it's much more fun to wave the nonlinearity boogeyman around: be afraid, be very afraid!