A litigant is someone involved in a lawsuit. The person who sues and the person who gets sued are both litigants.
To litigate is to use the legal system, and to be litigious is to be prone to filing lawsuits. Litigant refers to someone who is part of a lawsuit. If you sue a doctor, you’re a litigant, as is the doctor. If your landlord sues you, you’re both litigants. A company can be a litigant too. Litigants require lawyers. In fact, without litigants, lawyers would have nothing to do. Lucky for them, there are many litigants in the world.