Is "光轮" for "Nimbus" as in "Nimbus 2000 broomstick" just a transliteration? In a translation of the second Harry Potter book:

"All Harry's spellbooks, wand, cauldron and top-of-the-range Nimbus 2000 broomstick had been locked in a cupboard under the stairs by Uncle Vernon the instant Harry had come home."

哈利刚一到家，弗农姨父就把他的咒语书、魔杖、长袍、坩埚和最高级的光轮2000锁进了楼梯下那又小又暗 的柜子里 

It seems like this translation is well accepted: http://zh.harrypotter.wikia.com/wiki/%E5%85%89%E8%BD%AE2000?variant=zh

Google Translate's answer of 'light wheel' is unhelpful.

Is "光轮" just a transliteration or does it have extra meaning?
光轮 is not a transliteration, it is a translation

Halo (religious iconography)
A halo (from Greek ἅλως, halōs;1 also known as a nimbus, aureole

The round thing behind Buddha's head on that page is a 光轮 or 光环