What is a substitution phrase (murakkab badalī)?

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 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-khalīfatu ‘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-kitāba nisfahu), “I read the book, half of it”, نِصْفَه is a badal of a part from a whole — the speaker read half, not all.

Like most Arabic grammar terms, بَدَل (badal) was taken from everyday speech and then given one precise technical sense.

In everyday Arabic it means a replacement — something put in place of another.

بَدَلٌ مِنَ الْمَاءِ (badalun mina al-mā') — “a substitute for water”

In Arabic grammar it means a second naming that is the one actually meant.