The aminoacid which cannot be synthesizes by the body & 
therefore need to be supplied through protein rich foods in 
diet like sheep liver,egg,dairy products are called 
'essential aminoacids" or "Indespensable aminoacids".There 
are 10 essential aminoacids namely Mat,Arg,Thr, 
Trp,Val,Ile,Leu,Phe,His,Lys which are needed for human 
mostly in childhood.
      But His & Arg can be synthesized by adult humans.so 
these 2 aminoacids are considered as 'semi-essential 
aminoacids'