order of reaction
Imagine turning up a tap and watching how the flow responds. If doubling the opening doubles the flow, the response is 'first order'; if doubling it quadruples the flow, it is 'second order.' The order of reaction is the chemical version of that: a number that captures how strongly the reaction speed responds when you change a reactant's concentration.
Concretely, the order with respect to a given reactant is the power to which that reactant's concentration is raised in the experimental rate law. Add up the orders of all the reactants and you get the overall order. Because these powers come from experiment, they can be whole numbers, fractions, or even zero, and they need not match the coefficients in the balanced equation at all.
Order matters because it controls the whole shape of how a reaction unfolds in time — whether it has a constant half-life, slows in a particular curve, or stops abruptly. The key warning is that order is an empirical fact, not something you can deduce from the equation on paper; a fractional or unexpected order is actually valuable, because it hints at the multi-step mechanism underneath.
For rate = k[NO]²[H₂], the reaction is second order in nitric oxide, first order in hydrogen, and third order overall. Double the NO and the rate quadruples; double the hydrogen and it merely doubles — the orders tell you exactly how each knob turns the speed.
Orders quantify how each reactant's concentration steers the speed.
Order (an experimental number for the overall reaction) is not the same as molecularity (the count of particles in a single elementary step). They coincide only for elementary reactions.