Sentence:  One that we will consider here is the semiregular tessellation—a tessellation of two or more regular polygons that are arranged so that the same polygons appear in the same order around each vertex point. The tessellation at the left in Figure 9.38 is a <WSD>semiregular</WSD> tessellation because the two figures are a square and a regular octagon. The tessellation at the right in Figure 9.38 is not a <WSD>semiregular</WSD> tessellation because the two figures are a square and a nonregular octagon....There are only eight possible <WSD>semiregular</WSD> tessellations. 
Meaning of the word marked with <WSD> tags:  Uniform (isogonal and isotoxal) with regular faces of two or more types, such that each vertex is surrounded by the same polygons in the same order.