अदर्शनं लोपः

This defines elision. When a letter or word-form becomes latent, is neither heard, nor pronounced, nor written, it becomes lopa or is said to be elided. Lopa is the term for the disappearance of anything previously apparent.
In Sanskrit Grammar, this \lopa\ is considered as a substitute or adesa, and as such this grammatical zero has all the rights and liabilities of the thing which it replaces. This blank or lopa is in several places treated as having a real existence and rules are made applicable to it, in the same way as to any ordinary substitute that has an apparent form. The Grammarians do not content themselves with one sort of blank, but have invented several others; there are many kinds of them, such as lopa blank, slu blank, lup blank, and luk blank, which like different sorts of zeroes of a Mathematician, have different functions.

The word lopa occurs in sutras 6.1.66 and 6.4.118 &c. The lopa substitute is a sense substitute, and not a form substitute. Thus when we say 'let the substitute lopa take the place of such and such a letter or word,' we do not mean that the letters ल्, ओ, प् and अ should be substituted there, but the sense of the thing, namely 'disappearance.',