n.
   - a bar with a catch and lever used for fastening a door or gate.
 - a spring lock for an outer door, which catches when the door is closed and can only be opened from the outside with a key.
 - Electronics  a circuit which retains whatever output state results from a momentary input signal until reset by another signal.
 
v.
     - fasten with a latch.
 - Electronics  (of a device) become fixed in a particular state.
 - 
(latch on)
 informal understand the meaning of something.
 - 
(latch on to)
 informal attach oneself to (someone) as a constant companion. 
▸take up (an idea or trend) enthusiastically.
 - 
(latch on)
 (of a breastfeeding baby) manage to get its mouth into the correct position around the nipple.
 
 
  Phrase
  
    
      - on the latch
 chiefly Brit. (of a door or gate) closed but not locked. 
    
   
  Etymology
  OE læccan ‘take hold of, grasp’, of Gmc origin.