KEY: Efficacy.
SYN: Efficiency, competency, virtue, agency, capability, usefulness,energy, productiveness, effectiveness.
ANT: Inefficiency, inutility, ineffectiveness.

• Exculpate
• Excursion
• Excursive, [See DISCURSIVE]
• Excusable
• Excuse
• Execrable
• Execrate, [See ABOMINATE]
• Execration
• Execute, [See CONSUMMATE]
• Exemplary
• Exemplify
• Exempt
• Exemption
• Exercise
• Exercise \v
• Exertion
• Exhalation
• Exhale
• Exhaust
• Exhaustion, [See EXHAUST]
• Exhibit
• Exhibition
• Exhilarate
• Exhort
• Exhume, [See INTER]