A wicket is a little gate or door. In sports, wicket can refer to either the hoop you aim for when hitting a croquet ball or the stump you aim for when batting a cricket ball.
The very oldest meaning of wicket is "small door or gate," and it's still used this way sometimes, especially when the door is beside or within a larger one. This type of wicket is often found in a very old garden entrance or the door to a grand castle. Since the 17th century, the cricket meaning has become the most common, although North Americans are usually talking about croquet when they mention wickets.