What is Advocacy?

Legal Definition
Advocacy is an activity by an individual or group which aims to influence decisions within political, economic, and social systems and institutions. Advocacy can include many activities that a person or organization undertakes including media campaigns, public speaking, commissioning and publishing research or conducting exit poll or the filing of an amicus brief. Lobbying (often by lobby groups) is a form of advocacy where a direct approach is made to legislators on an issue which plays a significant role in modern politics. Research has started to address how advocacy groups in the United States and Canada are using social media to facilitate civic engagement and collective action.

An advocate is someone who provides advocacy support to people who need it.
-- Wikipedia
Legal Definition
The active support of an issue or the espousement of a set cause.