I tried to get a better idea of what al-qaza' meant, and after looking in a couple of dictionaries (Hans Wehr Compact and Lane's Lexicon), and looking through the ahadith I could find on the topic, it seems to me that what is meant is shaving the head, but leaving fluffy tufts of hair that look silly. This is my own
This hadith explained in more detail than others I found (there are quite a few shorter ones):
Narrated Ubaidullah bin Hafs:
that `Umar bin Nafi` told him that Nafi`, Maula `Abdullah had heard `Umar saying, "I heard Allah's Apostle forbidding Al-Qaza'." 'Ubaidullah added: I said, "What is Al-Qaza'?" 'Ubaidullah pointed (towards his head) to show us and added, "Nafi` said, 'It is when a boy has his head shaved leaving a tuft of hair here and a tuft of hair there." Ubaidullah pointed towards his forehead and the sides of his head. 'Ubaidullah was asked, "Does this apply to both girls and boys?" He said, "I don't know, but Nafi` said, 'The boy.'" 'Ubaidullah added, "I asked Nafi` again, and he said, 'As for leaving hair on the temples and the back part of the boy's head, there is no harm, but Al-Qaza' is to leave a tuft of hair on his forehead unshaved while there is no hair on the rest of his head, and also to leave hair on either side of his head.'"
حَدَّثَنِي مُحَمَّدٌ، قَالَ أَخْبَرَنِي مَخْلَدٌ، قَالَ أَخْبَرَنِي ابْنُ جُرَيْجٍ، قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ حَفْصٍ، أَنَّ عُمَرَ بْنَ نَافِعٍ، أَخْبَرَهُ عَنْ نَافِعٍ، مَوْلَى عَبْدِ اللَّهِ أَنَّهُ سَمِعَ ابْنَ عُمَرَ ـ رضى الله عنهما ـ يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَنْهَى عَنِ الْقَزَعِ. قَالَ عُبَيْدُ اللَّهِ قُلْتُ وَمَا الْقَزَعُ فَأَشَارَ لَنَا عُبَيْدُ اللَّهِ قَالَ إِذَا حَلَقَ الصَّبِيَّ وَتَرَكَ هَا هُنَا شَعَرَةً وَهَا هُنَا وَهَا هُنَا. فَأَشَارَ لَنَا عُبَيْدُ اللَّهِ إِلَى نَاصِيَتِهِ وَجَانِبَىْ رَأْسِهِ. قِيلَ لِعُبَيْدِ اللَّهِ فَالْجَارِيَةُ وَالْغُلاَمُ قَالَ لاَ أَدْرِي هَكَذَا قَالَ الصَّبِيِّ. قَالَ عُبَيْدُ اللَّهِ وَعَاوَدْتُهُ فَقَالَ أَمَّا الْقُصَّةُ وَالْقَفَا لِلْغُلاَمِ فَلاَ بَأْسَ بِهِمَا وَلَكِنَّ الْقَزَعَ أَنْ يُتْرَكَ بِنَاصِيَتِهِ شَعَرٌ، وَلَيْسَ فِي رَأْسِهِ غَيْرُهُ، وَكَذَلِكَ شَقُّ رَأْسِهِ هَذَا وَهَذَا.
|| : Sahih al-Bukhari 5920
|| : Book 77, Hadith 135
|USC-MSA web (English) reference
|| : Vol. 7, Book 72, Hadith 803
I also found this opinion on the Seeker's Hub site:
Answered by Shaykh Faraz A. Khan
Question: Is it permissible to get a fade haircut?
Answer: Assalamu alaikum wa rahmatullah, I pray this finds you in the best of health states.
To completely shave any part of the head and leave the rest unshaven is disliked, based on the hadiths in which the Messenger (peace and blessings be upon him) specifically prohibited doing so to one’s child. This prohibition would apply a fortiori to an adult. [Fatawa Hindiyya; Nahlawi, Durar Mubaha; Birgivi, Tariqa Muhammadiyya; Bukhari; Muslim]
Scholars are in unanimous agreement that such partial shaving is disliked: some deem the prohibition to indicate it being sinful, while others understand the prohibition to indicate that it is mildly disliked (makruh tanzihan) and not sinful per se. [Qari, Mirqat al-Mafatih]
Imam Birgivi mentions that the prohibition is due to the ugliness of such a look, and that it is based on the Prophet’s love for balance (peace be upon him), since partial shaving is a type of injustice to the head, leaving some of it covered and the rest bare. In that sense, it resembles the prophetic discouragement of wearing only one shoe. [Tariqa Muhammadiyya]
If part of the hair is not completely shaved but merely shorter than the rest, then it does not fall under the explicit prohibition of the hadith. However, if one does so specifically to imitate people who should not be imitated (such as celebrities or pop stars that display corrupt behavior), then it would be prohibited for that reason.
Some contemporary scholars are strict regarding such haircuts and do not allow them at all, due to the fact that imitation of corrupt people is usually the motivating factor. But if it is not, then it should be fine.
And Allah knows best.
Checked & Approved by Faraz Rabbani