2 – Invalidity (fāsid)
Linguistically: something that perishes in loss and ruin.
Technically: any act—whether devotional or contractual—whose legal consequences do not follow from its performance.

An invalid act of worship is one that neither clears liability nor fulfills the obligation, such as praying before its appointed time.

An invalid contract is one from which no legal effects ensue, like selling an unidentified object.

Every invalid form of worship, contract, or condition is prohibited, for it transgresses the limits set by Allah, treats His revelations with ridicule, and because the Prophet ﷺ censured those who stipulated conditions not found in the Book of Allah. (1)

The terms fāsid (invalid) and bāṭil (void) are synonymous except in two situations:

First: In iḥrām. They distinguish by calling it invalid (fāsid) when the pilgrim has sexual intercourse before the first release from iḥrām, and void (bāṭil) when he apostasizes while in that state.

Second: In marriage. They label as invalid (fāsid) any marriage whose invalidity is disputed among the scholars, such as a marriage contracted without a guardian, whereas void (bāṭil) refers to a union whose nullity is unanimously agreed upon, like marrying a woman still in her waiting period.