auxiliar

In obsolete terms the difference between auxiliar and assistant

is that auxiliar is auxiliary while assistant is someone who is present; a bystander, a witness.

As nouns the difference between auxiliar and auxilian

is that auxiliar is auxiliary while auxilian is a member of a group, typically composed of volunteers, that helps support a hospital.

As a noun auxiliar is

auxiliary.

As nouns the difference between ancillary and auxiliar

is that ancillary is something that serves an ancillary function, such as an easel for a painter while auxiliar is auxiliary.

As an adjective ancillary

is subordinate; secondary; auxiliary; accessory.