WordNet sense 1 (education or instruction in the fundamentals of a field of knowledge):

grounding

WordNet sense 2 (the fundamental assumptions from which something is begun or developed or calculated or explained):

base, threaten, cornerstone, groundwork, fundament

WordNet sense 3 (a woman's undergarment worn to give shape to the contours of the body):

foundation garment

WordNet sense 4 (lowest support of a structure):

foot, substructure, understructure

WordNet sense 5 (the act of starting something for the first time; introducing something new):

creation, initiation, innovation, instauration, institution, introduction, origination, founding