Jihadism is not a tactic, like terrorism, or a temperament, like radicalism or extremism. It is not a political pathology like Stalinism, a mental pathology like paranoia, or a social pathology like poverty. Rather, it is a religious ideology, and the religion it is associated with is Islam.
But it is by no means synonymous with Islam, which is much larger and contains many competing elements. Islam can be, and usually is, moderate; Jihadism, with a capital J, is inherently radical. If the Western and secular world's nearer-term war aim is to stymie the jihadists, its long-term aim must be to discredit Jihadism in the Muslim world.
No single definition prevails, but here is a good one: Jihadism engages in or supports the use of force to expand the rule of Islamic law. In other words, it is violent Islamic imperialism. It stands, as one scholar put it 90 years ago, for "the extension by force of arms of the authority of the Muslim state."
I generally view religion as a misguided foible, but when religion justifies violence against others, it passes from harmless ignorance to credible threat.