verb

• (transitive) To allow to, not to prevent (+ infinitive, but usually without to).
  "After he knocked for hours, I decided to let him come in."
• (transitive) To allow to be or do without interference; to not disturb or meddle with; to leave alone.
  "Let me be!"
• (transitive) To allow the release of (a fluid).
  "The physicians let about a pint of his blood, but to no avail."
• (transitive, chiefly British) To allow possession of (a property etc.) in exchange for rent.
  "I decided to let the farmhouse to a couple while I was working abroad."

verb

• (archaic) To hinder, prevent, impede, hamper, cumber; to obstruct (someone or something).
  "He who now letteth will let, until he be taken out of the way."
• (obsolete) To prevent someone from doing something; also to prevent something from happening.
  "& there was syr Mordred redy awaytynge vpon his londage to lette his owne fader to lāde vp the lande that he was kyng ouer. "And there was Sir Mordred ready awaiting upon his landing, to let his own father to land upon the land that he was king over.""
• (obsolete) To tarry or delay.
  "And for that strake I would not let, / Another upon him soon I set, […]"

noun

• (tennis) The hindrance caused by the net during serve, only if the ball falls legally.