Ṣalāt al-Tahajjud: — night prayer, pre-dawn prayer
“This bonus prayer is known as tahajjud prayer. It is not obligatory, but it is highly praised in God’s sight. After people go to bed and sleep for most of the night, they get up to pray a two-unit prayer an hour or two before the ﬁrst light of dawn appears. They can pray ( additional times as they wish. The Prophet made it his habit to pray the tahajjud prayer, and in time many other people began to join him.
Later on in Medina, he skipped doing it in the mosque one night, and this confused some people who had always looked forward to joining him. When they asked him about it the next moming, the Prophet explained that he skipped doing it in the mosque one night, because he was afraid it would be made an obligatory prayer on the community, (given how its practice was rising in popularity). He also counselled people to make tahajjud a regular habit or not to do it at all, rather than doing it only haphazardly with no regularity. A person who goes to bed with the intention of praying tahajjud later, but then sleeps through it inadvertently, still gets the reward of having done it. (N1sa’i, Ibn Majah)” (Emerick)
tahajjud: These are the voluntary late-night prayers. They are said between the performance of the Night (Isha) Prayer and the Dawn (Fajr) Prayer.