When two or more companies are merged with their assets and liabilities, they are called merger. Whereas when they are separated/detached from each other,they are called demerger.

In amalgamation two or more companies joint together to form a new company but in demerger one company splits itself into two or more new companies to work separately.

Yes, Centrica was formed in February 1997 following a demerger from British Gas plc.

The headquarters of Lucent Technologies is in Murray Hill, New Jersey, in the United States. This company was established in September 30, 1996 as a result of a demerger from AT & T Technologies.

In short 2002

Here is a snippit from Wikipedia's page on Montréal

"Montreal was merged with the 27 surrounding municipalities on the Island of Montreal on January 1, 2002. The merger created a unified city of Montreal which covered the entire island of Montreal. This move proved unpopular, and several former municipalities, totalling 13% of the population of the island, voted to leave the newly unified city in separate referendums in June 2004. The demerger took place on January 1, 2006, leaving 15 municipalities on the island, including Montreal." - http://en.wikipedia.org/wiki/Montreal