What is a substitution phrase (murakkab badali)?
In one line
Classical definition
Understand it
A word is named, then named again more precisely. The second naming is the بَدَل (badal); the first is the مُبْدَل مِنْه (mubdal minhu). Nothing stands between them — no particle, unlike the 'atf.
The two parts
The mubdal minhu is named first and prepares the way; the badal follows it and is the one actually meant.
In عَدَلَ الْخَلِيفَةُ عُمَرُ (‘adala al-khalifatu ‘umaru), “the caliph, Umar, was just”, عُمَر is the badal. The statement is really about Umar; الْخَلِيفَة only prepares the way for naming him.
The badal need not be the whole thing. In قَرَأْتُ الْكِتَابَ نِصْفَهُ (qara’tu al-kitaba nisfahu), “I read the book, half of it”, نِصْفَه is a badal of a part from a whole — the speaker read half, not all.
How to spot it
In the Qur'an
In the language — فِي اللُّغَة
The word بَدَل (badal) in everyday Arabic means a replacement — something put in place of another.
بَدَلٌ مِنَ الْمَاءِ (badalun mina al-ma') — “a substitute for water”
As a grammatical term it means a second naming that is the one actually meant.
Common questions
What is the difference between a badal and a description?
A description adds information about the noun. A badal replaces it — the second naming is the one the sentence actually means, and you could delete the first word and the sentence would still stand. That deletion test is the quickest way to tell them apart.
Updated