Al-Tahrimah: It is making something forbidden, and the opening Takbir is called Tahrimah because it forbids permissible things outside of prayer. It is a condition for the validity of commencing prayer; as stated by His Majesty: (And mentioned the name of his Lord and prayed) Al-A'la: 15, and He said: (And your Lord, glorify) Al-Muddathir: 3. Even if one enters prayer without Takbir, it does not require making up for it; because the commencement of the prayer is not valid in the first place, as Tahrimah is a condition for the validity of prayer, and if it is missed, the commencement is not valid. See: Maraqi Al-Falah p. 217.