In SCSS (Sass), mixins are a feature that allows you to define reusable blocks of CSS code. A mixin is similar to a function in programming, as it can accept parameters and generate CSS styles based on those parameters. Mixins provide a way to encapsulate and reuse common styles, making your code more modular and maintainable. To define a mixin in SCSS, you use the `@mixin` directive, followed by a name and a block of CSS code. Here's an example: 

@mixin center-element {
  display: flex;
  align-items: center;
  justify-content: center;
}
In this example, we define a mixin called `center-element` that applies common styles to center an element both vertically and horizontally using flexbox. To use a mixin, you can include it in a selector using the `@include` directive, followed by the name of the mixin. Here's an example: 

.container {
  @include center-element;
}
The `@include` directive includes the `center-element` mixin in the `.container` selector, which applies the styles defined in the mixin to that selector. After compilation, the generated CSS will include the styles from the mixin: 

.container {
  display: flex;
  align-items: center;
  justify-content: center;
}
Mixins can also accept parameters, allowing you to customize the generated styles. Here's an example of a mixin with parameters: 
@mixin link-color($color) {
  color: $color;
  text-decoration: none;
  
  &:hover {
    text-decoration: underline;
  }
}
In this example, the `link-color` mixin accepts a `$color` parameter. When using the mixin, you can pass a specific color value to customize the link's color. Here's an example: 

a {
  @include link-color(blue);
}
After compilation, the generated CSS for the `a` selector will include the customized color: 

a {
  color: blue;
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}
Mixins in SCSS provide a powerful way to reuse and share CSS code, making your stylesheets more maintainable and reducing code duplication. They are particularly useful for common styles or styles that require customization in different parts of your project.