Tafsir of Al-Ma'aarij 70:24

That is, a specific portion which they impose upon themselves as a means of drawing near to Allah Almighty and out of compassion for people. According to what has been narrated from Imam Abu Abd Allah—may Allah be pleased with him—it is what a man designates for himself to pay, for example, every Friday or every month.

It is also said that it is Zakat, because it is a determined, known amount. However, this is countered by the fact that the Surah is Meccan, whereas Zakat was only ordained and its amount specified in Medina; before that, it was indeed obligatory, but without a specified amount.