n. In botany, one of the three cardinal points of temperature — namely that point at which the metabolic processes are carried on with the greatest activity.n. The term is applicable not only to the factor of heat, but also to those of light, moisture, etc. The optimum for each several function must be distinguished from the total optimum for the plant, the most intense activity of a function often being injurious. According to Schimper, that condition which secures the highest intensity of a function is its absolute optimum; that which secures its most favorable activity is its harmonic optimum; and the harmonic optima together compose the œcological optimum for the plant.Best; most desirable.