Allaah azza wa jal mentions in his Noble Book:
"And stay in your houses, and do not display yourselves immorally (tabarruj) like that of the Days of Ignorance. And establish the prayer and give the Zakaat and obey Allaah and His Messenger" (Surah Al-Ahzaab:33)
Ibn Katheer (rahimahmullaah) said: Mujahid said: "A woman would go out and walk amongst men (i.e intermingling). This was the Tabarruj of the Day of ignorance.
Qatadah said: "The women would walk out, swaggering and flirting (with men) so Allaah forbade them from doing that"
Muqatil bin Hayyan said: "Tabarruj is when a woman puts a Khimar on her head but not fastening it properly so her necklaces, earrings and neck, and all of that can be seen. As a result of this, these are all exposed from her. This is Tabarruj"
(Mukhtasar Tafseer Ibn katheer, volume 3, page 94)
Imam Al-Qurtubee (rahimahullaah) said: "From the ways of Tabarruj is that a woman wears two transparent garments that describe her body"
(Tafseer Al-Qurtubee, volume 12, page 310)
Imam Abdur-Rahmaan Ibn Naasir As-Saiadee (rahimahullaah) said: This (ayah) means: Do not go out whilst you are beautified or perfumed, as the women of the Days of Ignorance used to do since they did not have any knowledge or religious characteristics.
(Tafseer Ibn Sa'adee, volume 6, no.107)
Imam Al-Albanee (rahimahullaah) said: "Tabarruj is when a woman exposes her adornment, body features and all that she is obligated to cover which invites the desires of men"
(Mar'at-il-Muslimah, page 54)
The Messenger of Allaah (may the peace and blessings of Allaah be upon him) said: "And the worst of your women are the ones who commit Tabarruj, who strut around in arrogance. They are the hypocrites. None from among them (i.e. these women) will enter Paradise except (those who are) like the rare crow."
(Reported by al-Bayhaqi in his Sunan 7/82) and authenticated by al-Albanee in his as-Saheehah (4/1849)
Commenting on this hadeeth, Imaam al-Albaanee (rahimahu Allah) said: "What is meant by the rare crow is that which has a red beak and red legs, as occurs in a hadeeth that states the small amount of women that will enter Paradise. (The hadeeth Imaam al'Albaanee is referring to is the one by 'Amr bin al-Aas who said: "One day we were with the Messenger of Allaah on this mountain path when he said: "Look! Do you see anything? We said: We see a crow that is very strange. Its beak and its legs are red." So Allaah´s Messenger said: " No one among will enter Paradise except for those who have the similitude of this crow in strangeness." (Reported by Ahmad 4/197 and Abu Ya'laa 1/349, and authenticated by al'Albaanee in as-Saheehah (4/1850).)
This is since there are very few crows that match this type of description." Silsilaat ul Ahadeeth as'Saheehah vol.4, no.1849.
From the Book: Four Essays of the Obligation of Veiling pages 90-91.