Ibn Manẓūr explains:
"Isrāf (الإسراف) refers to exceeding moderation.
To be extravagant with wealth (أسرف في ماله) means to spend hastily without due purpose.
The prohibited Isrāf, as mentioned by Allah, is spending wealth outside the obedience of Allah, whether it is a little or a lot.
Extravagance in spending is synonymous with Tabdhīr."
Reference: Lisān al-ʿArab (9/148)
Isrāf is defined as:
✨ "Using something in an improper manner, exceeding the appropriate limit."
Reference: Al-Kulliyyāt by Al-Kafawī (p. 113)
Al-Rāghib al-Aṣfahānī adds:
"Isrāf is the act of exceeding limits in any action performed by a person, although it is most commonly associated with overspending."
Reference: Al-Mufradāt fī Gharīb al-Qurʾān (p. 407)
Al-Jurjānī further clarifies:
"Isrāf means spending large amounts of wealth on lowly purposes.
It can also mean exceeding the proper bounds in spending, eating what is unlawful, or eating excessively even from lawful things, going beyond moderation and necessity.
Isrāf, therefore, involves a quantitative transgression, indicating ignorance of the proper measure of rights."
Reference: Al-Taʿrīfāt (p. 24)
Tabdhīr (التبذير) stems from the root meaning "scattering":
Originally, it referred to the throwing and dispersal of seeds (بذر).
The term was metaphorically extended to describe wasteful expenditure of wealth.
To "scatter" one's wealth (بذر ماله) means to spoil it and spend it wastefully.
Anything that is squandered and corrupted is said to be "badhara" (بذر).
The one who engages in wastefulness is called Mubadhdhir (مبذر).
Reference:
Al-Mufradāt fī Gharīb al-Qurʾān (p. 114)
Lisān al-ʿArab (9/148)
Imām al-Shāfiʿī defines:
✨ "Tabdhīr is the spending of wealth outside its rightful place."
Reference: Al-Jāmiʿ li-Aḥkām al-Qurʾān by Al-Qurṭubī (10/247)
Other scholars describe Tabdhīr as:
"Using resources for purposes where they are not appropriate."
"Dispersing wealth wastefully under the guise of extravagance."
References:
Al-Taʿrīfāt by Al-Jurjānī (p. 24)
Al-Kulliyyāt by Al-Kafawī (p. 113)
Al-Tawqīf ʿalā Muhimmāt al-Taʿārīf (p. 90)