Equilibrium
When chemists talk about equilibrium constants, they call reactions with large K values favorable and reactions with small K values unfavorable. It is important to note, however, these terms are not judgements about the value or usefulness of a particular chemical reaction. In fact, some very important reactions are actually "unfavorable"; one such example is the synthesis of ammonia from nitrogen and hydrogen gas (Haber process), which has a rather small equilibrium constant.