mahakAla - Great Time
Shiva Bhagawan is all-powerful, all-destroying time. nothing in this universe can escape the power of Time. Even the highest mountains are reduced to sand over time. Shiva Bhagawan as Mahakala is particulary associated with the city of Avantika (modern Ujjain) Just as Greenwich is the base meridian from which time is reckoned in the Modern system, per the Indian system Avantika is the meridien.
The decay and violence in this name and some of the others may suggest to some, that the Shaiva worship is very pessimistic and fearful. However, this would be mistaken. Shiva Bhagawan is not so much a destroyer but a regenerator. He destroys, not wantonly but only those impurities that must be cleared away for purity to flourish. The ultimate truth is beyond the reach of time and space and is not subject to creation, destruction etc.
The name mahaakaala indicates that He is kaalaatiita - transcends Time. There are legends about his having admonished yama (god of death), earning Him the epithets like KaalaantakaH, kaala-samhara-murtiH, etc.
Om mahakAlaya Namah