It's a relative term. First step is to eat food that's had as little processing as you can afford (which is the opposite of trans fats).

Ideally: if it's meat then non-intensive because that means less chemicals injected into the animals; if fruit and vegetables then try to source from farms that put an emphasis on re-mineralising their soils - they are less likely to rely on chemicals combined with genetically engineered varieties.

Essentially, buy food that makes the food industry as little profit as possible. The sort of food that would never be advertised on TV. The sort of food that costs a lot to produce, doesn't last long and lacks fancy packaging.