وَهُوَ تَأْدِيَةُ الْمَعْنَى بِعِبَارَةٍ نَاقِصَةٍ عَنْهُ مَعَ وَفَائِهَا بِالْغَرَضِ
Ijaz expresses a meaning with less wording than customary while still accomplishing the communicative purpose.
وَشَرْطُهُ وَفَاؤُهُ بِالْغَرَضِ
Its condition is that the expression must fulfil its purpose.
Brevity does not license removing essential meaning. It works through a concise but comprehensive expression or through an omission supported by adequate contextual evidence.
If the reader must speculate without a sufficient clue, the result is ikhlal—defective expression—rather than eloquent ijaz.