By diachronic approach we distinguish the primary meaning: table - a flat slab of stone and wood (ïëèòà), and secondary meanings (all the other meanings): ñòîë, ïèùà, òàáëèöà - as they are derived from the primary meaning of the word and appeared later than the primary meaning.
The terms «secondary» (âòîðè÷íîå) and «derived» (ïðîèçâîäíîå) meanings are to a certain extent synonymous. The term «secondary» denotes (implies) that this meaning appeared after the primary meaning.
When we refer to the meaning «derived» we do not only stress the fact that the meaning appeared after the primary one, but also that it is dependent on the primary meaning and somehow subordinate to it. E.g. in the case of the word table we may say that the meaning «food put on the table» is derived from the meaning «a piece of furniture on which meals are laid». As this particular meaning is derived through metonymic shift (change), we can also describe it as secondary and metonymic.
It follows that the main source of polysemy is a change in the semantic structure of the word.