Islamic midnight
The Islamic midnight is a special time connected to the prayer timings. It marks the end of the Isha prayer. Therefore, the Isha prayer should only be performed in case of necessity or if one has overslept.
The evidence in Sahih Muslim in the Hadith of 'Abdullah bin 'Amr states:
"We were informed by Abu Ghassan al-Misma'iy and Muhammad ibn al-Muthanna, who said: Mu'adh reported to us - and he was the son of Hisham - my father narrated to me, from Qatada, from Abu Ayyub, from Abdullah ibn Amr, that the Prophet of Allah (peace and blessings be upon him) said: 'When you perform the Fajr prayer, its time is until the first ray of the sun rises. Then, when you perform the Dhuhr prayer, its time is until the afternoon comes. When you perform the Asr prayer, its time is until the sun turns yellow. When you perform the Maghrib prayer, its time is until the twilight disappears. When you perform the Isha prayer, its time is until the middle of the night.'"
(Sahih Muslim, Chapter: The timings of the five prayers)
There are also less common opinions that the Isha prayer either ends with a third of the night or with the arrival of Fajr. However, to make the timing universally applicable regardless of the opinion followed, this page designates the Islamic midnight as the end. It is the earliest time and thus falls within the valid prayer time according to all three opinions.