The current meaning of militant does not usually refer to a registered soldier: it can be anyone who subscribes to the idea of using vigorous, sometimes extreme, activity to achieve an objective, usually political. A "militant activist" would be expected to be more confrontational and aggressive than an activist not described as militant.
Militance may or may not include physical violence, armed combat, terrorism, and the like. The Trotskyist Militant group in the United Kingdom published a newspaper, was active in labour disputes, moved resolutions in political meetings, but was not based on violence, although some confrontations might lead to unarmed fighting. The purpose of the Christian Church Militant is to struggle against sin, the devil and "..the rulers of the darkness of this world, against spiritual wickedness in high places" (Ephesians 6:12), but it is not a violent movement.
Militant can mean "vigorously active and aggressive, especially in support of a cause" as in 'militant reformers'. The American Heritage Dictionary of the English Language, defines militant as "Having a combative character; aggressive, especially in the service of a cause". The Merriam-Webster Dictionary defines militant as "aggressively active (as in a cause)". It says that the word militant might typically be used in phrases such as 'militant conservationists' or 'a militant attitude'.
An example of the adjective usages is demonstrated when The New York Times ran an article titled Militant Environmentalists Planning Summer Protests to Save Redwoods describing a group that believes in "confrontational demonstrations" and "nonviolent tactics" to get across their message of preserving the environment. Another usage example includes 'a militant political activist', drawing attention to behaviours typical of those engaged in intensive political activism. The political protests headed by the Reverend Al Sharpton have been described as militant in nature in The Washington Post.
The word "militant" is sometimes used to describe groups that do not name or describe themselves as militants, but that advocate extreme violence. In the early 21st Century, members of groups involved in Islamic terrorism such as Al-Qaeda are usually described as militants.