Perspex is a strong, clear plastic that can stand in for glass: you can see through it, but it does not shatter in the same way. In Britain the brand name became the everyday word, a bit like calling any tissue a Kleenex. In the United States people more often say Plexiglas, or just acrylic. You meet it in display cases, school windows, and light covers. It is a trade name that slipped into common speech, and in careful writing it may still take a capital P. Though actually, the thing itself is ordinary — in Taiwan you already know it as the sheets shops sell for signs and desks.