The Sexy Lamp test is a complement to the Bechdel test -- most would say an easier test, although it is possible to pass the Bechdel test and fail the Lamp test. It was originally put forth by Kelly Sue DeConnick, a popular American comic book writer/editor.
"I actually think the Bechdel Test is a little advanced for us sometimes. I have one called the Sexy Lamp Test, which is, if you can remove a female character from your plot and replace her with a sexy lamp and your story still works, you’re a hack."
This is in large part a reaction to the use of women as MacGuffins in adventure comics, but DeConnick is fairly serious in her writing, and has proposed a rule rather stronger than most people credit her with. It is often assumed that this refers specifically to the female lead character, and it certainly does... but the wording in the quote indicates that this rule is intended to apply to all female characters that are plot-relevant (we can assume that background actors are given a pass). This is essentially a statement that anyone important to the plot should have enough character development that they are distinguishable from an inanimate object.
Brevity Quest 2016