Key animation is the drawing of the defining poses of a movement, the frames that establish position, expression, and timing, which in-between animators then connect. The key animator of a cut is its principal author.
Animation is drawn in two layers of labour. Key animators draw the poses that define a shot: where a movement starts, where it peaks, and where it lands, plus the timing notes that say how long the shot has to get there. In-between animators then fill the frames between those poses so motion plays smoothly at the series' frame rate.
The key animator is therefore the author of a cut. Their drawing style survives the in-betweening, which is why experienced viewers can recognize a favourite animator's hand in seconds of screen time, and why key animation credits are followed so closely in sakuga culture.
The role structure explains a lot of anime's look. One animator may key an entire cut alone, giving it a unified hand; schedule pressure may split a cut awkwardly. When an episode feels uneven, the answer is usually in this credit list, in who keyed what and how much time they had to do it. A single celebrated key animator can be assigned a season's most important cut the way a studio assigns a star episode director, and fans plan their viewing around the credit.
In use"The leap, the landing, and the held expression were drawn by one key animator; assistants filled the frames between."