twbs--bootstrap-sass/vendor/assets/stylesheets/bootstrap/_component-animations.scss

// COMPONENT ANIMATIONS
// --------------------

.fade {
  opacity: 0;
  @include transition(opacity .15s linear);
  &.in {
    opacity: 1;
  }
}

.collapse {
  position:relative;
  height: 0;
  overflow:hidden;
  @include transition(height .35s ease);
  &.in {
    height: auto;
  }
}