The use and portrayal of the fan in Oscar Wilde's Lady Windermere's Fan is as varied as it is significant. A fancy communication tool mostly used in this specific time period, a lady's fan conveyed a language of its own combined with eye movements, hand movements, its placement, and even in the speed that it was waved. 

A lady would use her fan as a unique conduit of code language to convey messages to both men and women. A fan was an enforcer of communication. Arguably, the mere fact of holding the fan instilled in females almost the same level of comfort and security as a gun would have provided a man. In sum, the fan was a huge social facilitator because it was in itself a communicative device. Nowadays you can research anywhere in the Internet and find that "the language of the fan" was quite en-vogue during Wilde's time. 

This being said, the role of the fan in this particular play serves a similar purpose: the fan is often used by Wilde to foreshadow a situation, or to extend the meaning of Lady Windermere's expressions. The play begins on the day of Lady Windermere's "of- age" birthday. The fan is a gift form her husband, which she seems to cherish tremendously. 

Pretty, isn’t it!  It’s got my name on it, and everything...  It’s my husband’s birthday present to me...I’m of age to-day.  Quite an important day in my life.

However, as she learns about her husband's enigmatic monetary transactions with the mysterious Mrs. Erlynne, the fan becomes Lady Windermere's weapon of choice to bestow upon her potential rival the ultimate insult. 

[Picking up fan.]...  If that woman crosses my threshold, I shall strike her across the face with it.

As anger continues to invade Lady Windermere, she decides to cheat on her husband to get even. For this particular, she passes her fan to Lord Darlington in two separate occasions while saying

 A useful thing a fan, isn’t it? . . . I want a friend to-night, Lord Darlington: I didn’t know I would want one so soon.

Speak, sir!  Why is my wife’s fan here?  Answer me!  By God!  I’ll search your rooms, and if my wife’s here, I’ll -