Final answer:
The missing term is 'Progressive,' referencing the Populist and Progressive movements that advanced social causes in the late 1800s and early 1900s, aimed at reforming society and government.
Step-by-step explanation:
The Populist and Progressive movements were significant political developments in the United States during the late 19th and early 20th centuries, aimed at confronting social and economic problems of the era. Both movements sought to challenge the status quo and advocate for a variety of social causes. Populism, emerging in the 1890s, primarily represented the interests of farmers and pushed for financial reforms, direct election of senators, and other measures to counteract the influence of the economic elites. Progressivism, which gained momentum in the early 1900s, included a broader cross-section of society and focused on tackling issues such as workers' rights, government corruption, women's suffrage, and improving urban conditions. While the Populist Party dissolved as the 20th century began, many of their reforms were later adopted during the Progressive Era, leaving a lasting impact on American politics and society.