A disposition to behave in a certain way:
— The aptness of iron to rust.

synonym: propensity.

Roget 23: agreement; accord, accordance; unison, harmony; concord etc. 714; concordance, concert; understanding, mutual understanding.    conformity etc. 82; conformance; uniformity etc. 16; consonance, consentaneousness^†, consistency; congruity, congruence; keeping; congeniality; correspondence, parallelism, apposition, union.    fitness, aptness etc. adj.; relevancy; pertinence, pertinencey^†; sortance^†; case in point; aptitude, coaptation^†, propriety, applicability, admissibility, commensurability, compatibility; cognation etc. (relation) 9.    adaption^†, adjustment, graduation, accommodation; reconciliation, reconcilement; assimilation.    consent etc. (assent) 488; concurrence etc. 178; cooperation etc. 709.    right man in the right place, very thing; quite the thing, just the thing.    ... show more

Roget 176: tendency; aptness, aptitude; proneness, proclivity, bent, turn, tone, bias, set, leaning to, predisposition, inclination, propensity, susceptibility; conatus [Lat.], nisus [Lat.]; liability etc. 177; quality, nature, temperament; idiocrasy^†, idiosyncrasy; cast, vein, grain; humor, mood; drift etc. (direction) 278; conduciveness, conducement^†; applicability etc. (utility) 644; subservience etc. (instrumentality) 631.    ... show more

Roget 698: skill, skillfulness, address; dexterity, dexterousness; adroitness, expertness etc. adj.; proficiency, competence, technical competence, craft, callidity^†, facility, knack, trick, sleight; mastery, mastership, excellence, panurgy^†; ambidexterity, ambidextrousness^†; sleight of hand etc. (deception) 545.    seamanship, airmanship, marksmanship, horsemanship; rope-dancing.    accomplishment, acquirement, attainment; art, science; technicality, technology; practical knowledge, technical knowledge.    knowledge of the world, world wisdom, savoir faire [Fr.]; tact; mother wit etc. (sagacity) 498; discretion etc. (caution) 864; finesse; craftiness etc. (cunning) 702; management etc. (conduct) 692; self-help.    cleverness, talent, ability, ingenuity, capacity, parts, talents, faculty, endowment, forte, turn, gift, genius; intelligence etc. 498; sharpness, readiness etc. (activity) 682; invention etc. 515; aptness, aptitude; turn for, capacity for, genius for; felicity, capability, curiosa felicitas [Lat.], qualification, habilitation.    proficient etc. 700.    masterpiece, coup de maitre [Fr.], chef d'euvre [Fr.], tour de force; good stroke etc. (plan) 626.    ... show more

Appropriateness for the occasion:
— The phrase had considerable aptness.

synonym: appositeness.