restraint on alienation

: something that serves to prevent a party from alienating property; specifically : a provision in an instrument (as a deed or will) that purports to prohibit or penalize the use of the power of alienation : a provision in an instrument (as a deed or will) that purports to prohibit or penalize the use of the power of alienation

Note: Though not necessarily unlawful, restraints on alienation are disfavored in the law.