Rate Law: This is an equation that relates the rate of a reaction to the concentrations of its reactants. For first order reactions, it shows that rate is directly proportional to one reactant's concentration.
Half-Life: In context of first order reactions, this refers to the time it takes for half of a given amount of reactant to be consumed.
Rate Constant: This is a proportionality constant in the rate law which reflects how quickly or slowly a reaction occurs.