one-to-one
Concise Oxford English Dictionary © 2008 Oxford University Press:
one-to-one (also chiefly N. Amer. one-on-one)
▶adjective & adverb denoting or referring to a situation in which two parties come into direct contact or opposition.
■ Mathematics in which each member of one set is associated with one member of another.
▶noun informal a face-to-face encounter.'one-to-one' also found in these Oxford entries:

