Jack:

A number eleven batter. Derived from the jack playing card, which ranks immediately after the number ten in each suit.

Jaffa (also Corker):

An exceptionally well-bowled, practically unplayable delivery, usually from a fast bowler.

Jayadevan’s System:

An unsuccessful proposal for a rain rule, an alternative to the Duckworth-Lewis system, never used in professional cricket.

Jockstrap (also Jock Strap):

Underwear for male cricketers designed to securely hold a cricket box in place when batting or wicket-keeping.

Just Not Cricket:

See “It’s (Just) Not Cricket.”