sadasadbāhyaṃ vastu nibandhanamakṣaracihnasthānīyaṃ svarūpamupadarśayituṃ prakamyate yasya, sa bāhyavastunibandhanaḥ
That which has the real-unreal external object as its basis, standing in place of letter-symbols, whose form is intended to be manifested - that is [called] "based on an external object."