Bull market is a colloquial term to indicate a period of optimism in a financial market when the value of its constituents are consistently either going up or expected to go up.
In the stock market, a bull market is when its equity share prices are rising or expected to rise. The term can be applied to such runs in real estate, gold and other commodity markets as well.
Since financial markets are volatile, a short-term rise in the markets should not be termed as a bull market; only growth for a sustained period in the market is a bull market. But traders have been known to jump to quick conclusions and use the term a little too soon.