Final answer:
An early American polyphonic setting of a psalm or hymn in an imitative style is referred to as an Anthem, prevalent during the 18th and 19th centuries and significantly linked to the tradition of lined-out hymns and the spreading of religious teachings in communities.
Step-by-step explanation:
An early American polyphonic setting of a psalm or hymn in an imitative style is called an Anthem. During the 18th and 19th centuries, as missionaries such as Baptist, Methodist, and Presbyterian spread the gospel, hymns and psalms were a significant way of teaching and preserving religious teachings. Notable hymnwriters like Isaac Watts contributed to the tradition of lined-out hymns or line singing, which were an essential part of worship in many communities, including African American slave communities. This method, also related to the call and response of African musical traditions, was particularly useful where churchgoers often could not read or afford hymnals. The practice, which involved a leader singing or intoning lines of a hymn followed by the congregation repeating it back, led to a unique musical expression that became foundational to other genres like the blues.