«قاصر: وهو تسليم عين الواجب مع النقصان في صفته»
Deficient ada renders the required act but with an imperfection in its manner or attributes.
The book includes omission of al-Fatiha as an example of deficiency in prayer, and also mentions improper alteration of pillars. Those examples must not be given a single undifferentiated ruling: omission of a required quality and loss of an essential pillar differ.
The book’s principle is to remedy a remediable deficiency where possible. When an essential constituent is absent, merely treating the act as a minor imperfection is not enough.