Terminal phosphate linkage means

ATP is made unstable by the three adjacent negative charges in its phosphate tail, which "want" very badly to get further away from each other. The bonds between the phosphate groups are called phosphoanhydride bonds, and you may hear them referred to as “high-energy” bonds. ATP is formed of adenine, ribose and a row of three phosphate radicals attached to ribose. The last two phosphate radicals are attached by bonds of high transfer potential. This bond is also known as terminal phosphate linkage. This bond can be very easily synthesised and broken up. ATP is the higher energy form (the recharged battery) while ADP is the lower energy form (the used battery). When the terminal (third) phosphate is cut loose, ATP becomes ADP (Adenosine diphosphate; di= two), and the stored energy is released for some biological process to utilize.