latch

1 latch /ˈlætʃ/ noun
plural latches
1 latch
/ˈlætʃ/
noun
plural latches
Britannica Dictionary definition of LATCH
[count]
1 
: a device that holds a door, gate, or window closed and that consists of a bar that falls into a holder when it is closed and that is lifted when it is open
2 
chiefly British : a type of door lock that can be opened from the inside by turning a lever or knob but can only be opened from the outside with a key

on the latch

British, of a door
: closed but not locked
2 latch /ˈlætʃ/ verb
latches; latched; latching
2 latch
/ˈlætʃ/
verb
latches; latched; latching
Britannica Dictionary definition of LATCH
[+ object]
: to close or fasten (something, such as a door) with a latch
3 latch /ˈlætʃ/ verb
latches; latched; latching
3 latch
/ˈlætʃ/
verb
latches; latched; latching
Britannica Dictionary definition of LATCH

latch on

[phrasal verb] informal
1 
British : to begin to understand something
2 
latch on to (something) or latch onto (something)
a  : to grab and hold (something) — often used figuratively
b  : to begin using or doing (something) in an enthusiastic way
: to choose (something) in an enthusiastic way
3 
latch on to (someone or something) or latch onto (someone or something) : to stay close to (someone or something)