Longman Dictionary of Contemporary English - jam

~1 n 1 a very thick sweet substance made from boiled fruit and sugar and eaten especially on bread; conserve  (strawberry jam) 2 a situation in which it is difficult or impossible to move because there are so many people, things, cars etc close together  (traffic jam)  (Sorry we're late. We got stuck in a traffic jam.) 3 be in a jam informal to be in a difficult or uncomfortable situation 4 jams AmE brightly coloured trousers that stop above your knee 5 jam tomorrow BrE informal good things that you are promised if you are patient enough to wait for them  (- see also jam session) ~2 v 1 »PUSH HARD« to push something somewhere using a lot of force, or to push too many things into a small place  (jam sth into/under/on)  (I'll never jam all my clothes into one suitcase. | be jammed (up) against sth (=pushed tightly against something)) 2 »MACHINE« also jam up if a lock or a moving part of a machine jams, or if you jam it, it no longer works properly because something is preventing it from moving  (The front roller has jammed on the photocopier.) 3 »BLOCK« also jam up if a lot of people or vehicles jam a place, they block it so that it is difficult to move  (Crowds jammed the entrance to the stadium.)  (- see also jammed (2)) 4 »MUSIC« to play music informally with others without practising first  (- see also jam session) 5 jam on the brakes to slow down a car suddenly by putting your foot down hard on the brake1 (1) 6 jam the switchboard if telephone calls jam the switchboard, so many people are telephoning the same organization that its telephone system cannot work properly 7 »RADIO« to deliberately prevent broadcasts or other electronic signals from being received by sending out noise on the same wavelength 8 sb is jamming AmE spoken used to say that someone is doing something well