The definition of assimilationism is the act of encouraging minority cultural groups to be similar to larger cultural groups.
An example of assimilationism is the encouragement by some Americans for those who come to the United States to dress in a similar fashion as Americans.
the policy of completely absorbing minority cultural groups into the main cultural body, esp. by intermarriage
A policy of furthering cultural or racial assimilation.
From assimilation + -ism.