1, The Full Form of CTU is Counter Terrorist Unit.

The Counter Terrorist Unit (also referred to as the Counter Terrorism Unit or CTU) was an intelligence and law enforcement agency in the United States of America which worked to investigate and prevent terrorist acts within U.S. borders. CTU had divisions established in cities all over the country and had prevented several imminent terrorist attacks on U.S. soil.

After Day 6, CTU was disbanded following the supposed extralegal activities of several agents and was under investigation for human rights violations. Its functions were then delegated to other agencies, and the personnel had their government clearance revoked. However, following Day 7, CTU was resurrected with the help of Rob Weiss, who convinced President Taylor to reestablish the agency in full capacity.

2, The Full Form of CTU is Coding tree unit

Coding tree unit (CTU) is the basic processing unit of the High Efficiency Video Coding (HEVC) video standard and conceptually corresponds in structure to macroblock units that were used in several previous video standards. CTU is also referred to as largest coding unit (LCU).

A CTU can be between 16×16 pixels and 64×64 pixels in size with a larger size usually increasing coding efficiency. The first video standard that uses CTUs is HEVC/H.265 which became an ITU-T standard on April 13, 2013

see also What does LCU mean? What is the full form of LCU?