A wormhole was a stable gravitic anomaly that caused a flaw in normal space providing access through hyperspace between widely separated points.
They normally consisted of a wormhole junction and its associated termini, but could also consist of only two termini forming a wormhole bridge or "hyper bridge". (HH13)
The first wormhole was discovered in 1447 PD. (infodump)
The largest junction known to exist was the Manticore Wormhole Junction, with seven termini discovered as of 1922 PD. (HH1, SK1)