noun

• (chiefly uncountable) Capacity of mind, especially to understand principles, truths, facts or meanings, acquire knowledge, and apply it to practice; the ability to comprehend and learn; the ability to process sentient experience to generate true beliefs with a justified degree of confidence.
• (chiefly uncountable) The quality of making use or having made use of such capacities: depth of understanding, mental quickness.
• (countable) An entity that has such capacities.
• (chiefly uncountable) Information, often secret, about an enemy or about hostile activities.
• (countable) A political or military department, agency or unit designed to gather information, usually secret, about the enemy or about hostile activities.
• (dated) Acquaintance; intercourse; familiarity.