A case when one of two similar things is incorporated in the other, which makes one of them sufficient. If two things of the same type are available and their purposes are the same, one of them will mostly be incorporated in the other.
المعنى الاصطلاحي :
Merging two or more acts of worship of the same kind.
الشرح المختصر :
"Tadākhul" (overlapping) generally means two things overlapping without any increase in size or amount. An example of "tadākhul" in worship is "tadākhul" in purification. If a person is liable to two or more obligatory acts, one of them is merged into the other, such as when a woman has to make "ghusl" from sexual impurity and "ghusl" after the end of her menstruation. There are two conditions for the validity of "tadākhul": 1. The two acts must be of the same kind, such as both being prayers or both being acts of purification, etc. Also, if a person has to do two acts at the same time, and one is obligatory while the other is recommended, such as "tahiyyat al-masjid" (prayer to greet the mosque) and the Zhuhr prayer; so the two acts merge or overlap. If they are of different levels, then the lower one merges into the higher one, and doing the higher one is sufficient to fulfill both acts. If they are of the same level, then doing one of them is sufficient. 2. One of them should not be intended in itself, such as the case of two obligatory prayers; "tadākhul" cannot be done in this case, and each prayer must be performed separately.
التعريف اللغوي المختصر :
"Tadākhul": mixing, intertwining . Original meaning: one thing entering into another or merging with it.