To set the name of the CSS property the transition effect is, use the CSS transition-property.
In the below example, we have set the property as width and set the duration as well:
Example
<!DOCTYPE html>
<html>
<head>
<style>
div {
width: 150px;
height: 150px;
background: blue;
transition-property: width;
transition-duration: 3s;
}
div:hover {
width: 250px;
}
</style>
</head>
<body>
<h1>Heading One</h1>
<p>Hover over the below box to change its width.</p>
<div></div>
</body>
</html>