An appearance is the act of a defendant showing up in court. The defendant does not have to make the appearance themselves in all cases; many times, lawyers will appear on behalf of their clients. The act of making an appearance is an admission of the court's authority to proceed. Appearances can be done at any time throughout a court matter.
If a defendant does not agree that they should be sued, for whatever reason, they may show up in court to make a point. This would be called making a special appearance, as the person does not confirm the court's authority to proceed at that stage. Additionally, a person might not agree to make a full appearance at a court case for a variety of other reasons, such as if the individual was not properly served with the court papers in the first instance. In these cases, the person will make a special appearance in order to gain the correct paperwork, and then at the next court sitting, the person will make their formal appearance. Although defendants in both civil and criminal cases have to appear in court, the term appearance is more often used in criminal law cases.