David Broodryk has written a good small piece on what a movement is at http://davidbroodryk.org/index.php/blog/53-what-is-a-movement.
I, too, think of a movement not in terms of a “black-and-white definition” but rather core concepts that help to identify the “fuzzy boundaries” that are around most movements. The core concepts are pretty much the same as what David identifies. “Reproduction” is key. Another is “Generations.” We use the CPM Continuum scale, which, roughly, is:
0 CPM Team in context but no purposeful CPM plan or efforts yet
1 Moving purposefully – Trying to consistently establish 1st generation of NEW believers & churches
1.1 Purposeful Field 1 and Field 2 (entry – looking for person of peace / houses of peace — and evangelism) activity but no results yet
1.2 Have some new Gen1 believers
1.3 Have some new Gen1 believers and new groups
1.4 Have consistent new G1 believers
1.5 Have consistent new G1 believers and new groups
1.6 One or more new first generation churches
1.7 Several new G1 churches
1.8 G1 churches are starting new groups
1.9 Close to G2 churches (1+ G2 church)
2 Focused – Some 2nd gen churches (i.e. new believers/churches have started another generation)
3 Breakthrough – Consistent 2nd generation and some 3rd gen churches
4 Emerging CPM – Consistent 3rd gen churches and some 4th gen churches
5 CPM – consistent 4th++ generation churches in multiple streams
6 Sustained CPM – Visionary, indigenous leadership leading the movement with little/no need for outsiders. Stood test of time with at least several hundred churches (Most stage 6 CPMs have 1000 or more churches)
7 Multiplying CPMs – Initial CPM is starting to catalyze other CPMs
Another scale I’ve been toying with is the rough size of a movement. People rarely know precisely how big a movement is, but it’s usually possible to measure in terms of order of magnitude. I’ve used many different scales over the years; I usually try to capture both rounded estimates and a “scale” entry.