In morpheme-based morphology, a null morpheme is a morpheme that is realized by a phonologically null affix (an empty string of phonological segments). In simpler terms, a null morpheme is an "invisible" affix. It is also called a zero morpheme; the process of adding a null morpheme is called null affixation, null derivation or zero derivation. The concept was first used over two thousand years ago by 4th century Sanskrit grammarian from ancient India, Pāṇini, in his Sanskrit grammar. Some linguists object to the notion of a null morpheme, arguing that it sets up an unverifiable distinction between a "null" or "zero" element, and nothing at all.