Sentence:  In this chapter we give some examples of Fuchsian <WSD>groups</WSD>. The most interesting and important ones are the so-called "arithmetic" Fuchsian <WSD>groups</WSD>, i.e., discrete subgroups of PSL(2,<WSD>R</WSD>) obtained by some "arithmetic" operations. One such construction we have already seen: if we choose all matrices of SL(2,<WSD>R</WSD>) with integer coefficients, then the corresponding elements of PSL(2,<WSD>R</WSD>) form the "modular <WSD>group</WSD>" PSL(2,Z). 
Meaning of the word marked with <WSD> tags:  A set with an associative binary operation, under which there exists an identity element, and such that each element has an inverse.