Sellotape is the British name for that thin, clear, shiny tape that is sticky on one side. You tear off a strip to stick paper, wrap a present, or mend a torn page. In Britain the brand name became the everyday word, the way people say a hoover for any vacuum cleaner — though actually the company is just one maker, and the roll on your desk might be another brand. In American English people usually say Scotch tape, which is also a brand used as a common word. You can use it as a verb too: to sellotape a box shut. It is informal household English, not a science word.