rdzeń.

pith, medulla (soft tissue filling the root, stem, or trunk of a tree)

core (most important part of a whole)

core (long middle part of cylindrical objects)

cord (part of the nervous system located in the spinal canal connecting to the brain)

root (morpheme containing the basic meaning of a word, found in all words related to it)

core (metal part placed inside an electromagnet or transformer)

core (individual computer processor, in the sense when several processors are plugged together in one single integrated circuit to work as one)

core (main part of a nuclear reactor containing uranium)

core (part of a casting mold that allows the internal shape of an object to be recreated)

core (hardest part of an abscess, boil, corn, etc.)

core (center or middle part of something)