Bullish long-term trading
When an investor is bullish on a company for the long term, it means they have a favorable view of the company's future. They may also believe the stock is currently undervalued at its current share price.
The term could also be applied to a sector, industry, or the viability of a technology. For example, someone might say they're bullish on brick-and-mortar retail or autonomous vehicles. An investor who is bullish on an entire industry may invest in several companies that participate in the sector in the hope of finding the eventual market leader.
Bullish short-term trading
If a short-term trader is bullish, they believe a stock will go up in the coming days, weeks, or even minutes. This may be based on analyzing stock charts or intraday volume and price action. In these cases, the bullish viewpoint may have nothing to do with the underlying company. For instance, if a trader believes a stock is oversold, they may buy shares in the hope of a quick reversal.
Other short-term traders are bullish because they're betting some near-term event will happen in a favorable manner. For example, a trader may buy a stock the day before its quarterly earnings are released, hoping that the company will beat expectations.