A Secular market stretches over many years and likely will contain several up and down market cycles. Another adjective really is needed in the phrase to make clear what type of market (bull or bear, for example) the secular market is or has been.
Timing isn't everything in investing, but it can feel like it sometimes. Sell your shares today; you grab a 20% return. Sell next week and rake in 30%. Will the stock market turn to your advantage if you hang in for a couple of years? Is now the time to ditch your ownership in a company?
There's a lot to consider, but the handy-dandy bear and bull are a shorthand way to get a quick feel of the pulse of investors. But it's no science; not even a 'dismal science'. It might be more like the 'sweet science' of boxing, where there are opposing sides, a panel of judges who are free to disagree, and participants who can get knocked down yet still win the fight.
Whether the market is bear or bull depends on the time frame you are considering. It can be bullish today, even if the overall trend is bearish. Some might say we're in a bear market while others give the decision to the bull. A secular market stretches some 5-20 years. There's an overall market trend (up or down) that isn't diminished by opposing trends within the time.
Wars and demographic changes are examples of long-lasting forces that create the overall tone of the secular market.