What's the term used to describe a figure who comes to represent a movement in the common perception of the population?
I don't mean a leader or a spokesperson, I'm meaning more that they're a mascot for the movement.
icon. from The Free Dictionary (TFD)
a person or thing regarded as a symbol of a belief, nation, community, or cultural movement
The word icon originally had a deep religious significance, being (TFD)
A representation or picture of a sacred or sanctified Christian personage, traditionally used and venerated in the Eastern Church.
Rosa Parks has an almost religious significance for the Civil Rights Movement and for many of her admirers.
The words icon or iconic are used much more broadly, for example a fashion magazine might describe a simple little black dress (wildly overpriced) iconic. And Icon in Computerese means "a pictorial representation of a facility available on a computer system" (TFD), but none of these meanings should detract from the more profound meaning that Rosa Parks exemplifies.
Rosa Parks not only launched this new paradigm but incorporated all those that preceded it: Old Leftism, New Deal liberalism, unionism, NAACP legalism and gradualism. She was an embodiment of the civil rights movement to that moment, even if the impression persists that she was a simple old lady with aching feet.
Rosa Parks: The story behind her sitting down - By Diane McWhorter - Slate Magazine, 2005. Emphasis mine.
Someone or something that is a perfect representative or example of a quality, idea, etc.
"Pioneer", "hero", "champion", or "role model" perhaps could fit also.
- a person who is among those who first enter or settle a region, thus opening it for occupation and development by others.
- one who is first or among the earliest in any field of inquiry, enterprise, or progress.
She was certainly one of the first activists in the Civil Rights Movement.
- A person noted for feats of courage or nobility of purpose, especially one who has risked or sacrificed his or her life
She demonstrated courage and risked punishment for her civil disobedience.
- An ardent defender or supporter of a cause or another person
Her actions were not just for her own self-interests.
A person who serves as a model in a particular behavioral or social role for another person to emulate.
And she served as a model for other acts of civil disobedience.
How about emblem, symbol, harbinger, or herald?
emblem: a person or thing that represents an idea
symbol: an action, object, event, etc., that expresses or represents a particular idea or quality
These seem rather close to the OP's "mascot".
The following also apply to Rosa Parks, but they might not be as close to the word the OP is seeking:
harbinger: one that pioneers in or initiates a major change; one that presages or foreshadows what is to come
herald: one that precedes or foreshadows
"Rosa Parks was a harbinger of the civil rights movement."
"Rosa Parks was a herald of the civil rights movement."
I'd like to suggest torchbearer as in the following sentence: In refusing to relinquish her seat, Rosa Parks became a torchbearer for the American Civil Rights Movement.
torchbearer: Someone in the forefront of a campaign, crusade, or movement.
In the sense that the term suggests no actual power, as the person is only perceived as the leader of a movement, I am thinking of figurehead:
a person who is called the head or chief of something but who has no real power
The figurehead originally was the carved figure on a ship's bow.
If a single person is synonymous with an idea, they can be described as the face of that idea. It suggests that their physical appearance is so well-known that simply observing it evokes thoughts of the idea they represent. Likewise, it suggests that thinking of the idea itself invariably evokes imagery of that person's physical appearance.
I've written this example to demonstrate:
One picture of Rosa Parks is all it takes to remind us of how far we've come. She's truly the face of the American civil rights movement.
Folks are factoring the meaning too much. She is both an embodiment and a 'poster-child' so she is an:
exemplar - a person or thing serving as a typical example or excellent model.
paragon - a person or thing regarded as a perfect example of a particular quality.
As a native speaker, "hero" and/or "poster child" sound best to me.
"Hero" doesn't imply uniqueness. She could be one of many. There's usually only one "mascot" for a team or cause.
"Poster child" suggests uniqueness, but it also doesn't have the positive connotation of "hero". Someone can be the "poster child" for something entirely negative.
"Figurehead" is bad because it has negative implications. It brings to mind a leader who isn't really a leader; someone else is really in charge.
Several of the others ("symbol", "embodiment", "mascot") usually aren't used to refer to specific individuals. "Mascot" in particular, when applied to Parks, sounds vaguely insulting. As if she were cynically used to promote the cause. If that's the meaning you're trying to convey then great, but otherwise I'd avoid "mascot".