An Ultrabook is a laptop that is very thin, very light, and still fast — the kind you slide into a bag and forget is there. The important thing your dictionary may not tell you: this is a brand word, not a plain English word. Intel made it up around 2011 and set rules — a laptop could only be called an Ultrabook if it was under a certain thickness, woke up from sleep within a few seconds, ran a certain grade of Intel chip, and so on. Makers who met the rules could put the name on the box. That is why you often see it written with a capital U, and why it was everywhere in computer shops for a few years. Two things follow from that. First, ordinary people never kept the rules straight — most speakers just use "ultrabook" loosely for any slim, expensive-looking laptop, the MacBook Air shape, even when Intel would say it does not qualify. Second, the word is now dated. The marketing push ended, and today people simply say "a thin laptop", "a lightweight laptop", or name the machine. If you use it in conversation you will be understood, but you may sound like you are reading a 2013 advertisement. It is a countable noun: an Ultrabook, two Ultrabooks.

超輕薄筆電（Intel 的品牌名；台灣店裡多半直接講「輕薄筆電」或講機型名，很少人真的說「超極本」——那是中國的譯法）