WordNet sense 1 (the central meaning or theme of a speech or literary work):

core, effect, essence, gist

WordNet sense 2 (an onerous or difficult concern):

encumbrance, load, onus, incumbrance

WordNet sense 3 (weight to be borne or conveyed):

loading

WordNet sense 4 (weight down with a load):

weight, burthen, weight down

WordNet sense 5 (impose a task upon, assign a responsibility to):

charge, saddle