1. [n]  catch for fastening a door or gate; a bar that can be lowered or slid into a groove
2. [n]  spring-loaded doorlock that can only be opened from the outside with a key
3. [v]  fasten with a latch

1. \Latch\, v. t. [Cf. F. l['e]cher to lick (of German
   origin). Cf. {Lick}.]
   To smear; to anoint. [Obs.] --Shak.
   
   

2. \Latch\, n. [OE. lacche, fr. lacchen to seize, As.
   l[ae]ccan.]
   1. That which fastens or holds; a lace; a snare. [Obs.]
      --Rom. of R.
   
   2. A movable piece which holds anything in place by entering
      a notch or cavity; specifically, the catch which holds a
      door or gate when closed, though it be not bolted.
   
   3. (Naut.) A latching.
   
   4. A crossbow. [Obs.] --Wright.
   
   

3. \Latch\, v. t. [imp. & p. p. {Latched}; p. pr. & vb. n.
   {Latching}.] [OE. lacchen. See {Latch}. n.]
   1. To catch so as to hold. [Obs.]
   
            Those that remained threw darts at our men, and
            latching our darts, sent them again at us.
                                                  --Golding.
   
   2. To catch or fasten by means of a latch.
   
            The door was only latched.            --Locke.
   
   

A digital logic circuit used to store one or more bits. A latch has a data input, a clock input and an output. When the clock input is active, data on the input is "latched" or stored and transfered to the output either immediately or when the clock input goes inactive. The output will then retain its value until the clock goes active again.

See also flip-flop.