optimum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The point at which the condition, degree, or amount of something is the most favorable.
2. n. Biology The most favorable condition for growth and reproduction.
3. adj. Most favorable or advantageous; best.

Century Dictionary and Cyclopedia

1. 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. “The minimum or zero point is the point at which the performance is just possible; the optimum point, at which it is carried on with the greatest activity; and the maximum point, at which it is arrested.” (Vines.)
2. 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.
3. Best; most desirable.

Wiktionary

1. n. The best or most favorable condition, or the greatest amount or degree possible under specific sets of comparable circumstances.
2. adj. best or most advantageous; surpassing all others

GNU Webster's 1913

1. adj. Best possible or most desirable; -- usually under a restriction expressed or implied.
2. n. The most favorable condition, greatest degree, or largest amount possible under given circumstances.

WordNet 3.0

1. n. most favorable conditions or greatest degree or amount possible under given circumstances
2. adj. most desirable possible under a restriction expressed or implied