Final answer:
The term for a 19th-century U.S. antislavery advocate is 'abolitionist'. They pushed for the immediate end to slavery and included influential figures such as William Lloyd Garrison and Frederick Douglass.
Step-by-step explanation:
The general term for a 19th-century U.S. antislavery advocate is abolitionist. Abolitionists were believers in the complete elimination of slavery, and they included a variety of individuals such as prominent journalists, religious leaders, and former slaves. Their efforts were diverse, including moral suasion, political campaigns, and assisting runaway slaves. Influential figures like William Lloyd Garrison and Frederick Douglass led the movement with their compelling writing and speaking. Though a minority, abolitionists had a significant effect on American society by highlighting the immorality of slavery and pushing for immediate emancipation, also known as immediatism.