Oxford English Dictionary. Assimilate. 2.a. To be or become like to, resemble. . . . 7.a. To convert into a substance of its own nature, as the bodily organs convert food into blood, and thence into animal tissue; to take in and appropriate as nourishment; to absorb into the system, incorporate. 8.a. To become of the same substance; to become absorbed or incorporated into the system.
Assimilation. 1.a. The action of making or becoming like; the state of being like; similarity, resemblance, likeness. . . . 4.a. Conversion into a similar substance
To assimilate means to become similar. For immigrants or minorities, assimilation is a process by which distinct ethnic groups become more like other Americans, especially in terms set by the USA's dominant culture. (As America diversifies, however, so do the terms of assimilation.)
As immigrants learn a new nation's common language, intermarry, participate in shared institutions like public schools and beliefs like opportunity or individualism, their unique ethnic or cultural differences tend to diminish or disappear. (Resistance to such disappearance is a strong interest of multiculturalism.)
But assimilation can work both ways: the dominant culture sometimes absorbs or admires values, practices, and products of immigrants or other ethnic groups:
extended or stable families ("America stresses the family.")
Acculturation is sometimes preferred, maybe because the word is more ambiguous and unfamiliar, but also because it can imply "selective assimilation."
Instructor's attitude is that some forms of assimilation are always in process (or being resisted), so we may as well analyze and discuss.
Processes of assimilation:
education (esp. public schools)
intermarriage (which erases or moderate physical differences like skin color, eye shape, hair texture, etc.)
appearance, fashion (> plain style?)
cleanliness, disinfection? (i.e. "soap and water")
Institutions of assimilation:
public schools (b/c less segregated by race—at least officially—though increasingly segregated by class and location)
military (highest rate of racial intermarriage; plus inculcation of dominant-culture systematic discipline, chain-of-command, etc.)
popular culture (representation and consumption)
Institutions averse to assimilation:
religious schools of any creed, bible academies (sometimes called "segregation academies," though conversion to Evangelical Christianity may gain entrance for ethnic outsiders)
private clubs (e.g. segregated golf courses, bars requiring membership invitations)
In post-Civil Rights America, segregation is no longer legal, but it has survived and even grown with increasing inequality of wealth and income + white flight.