Meat, in a broadest sense, means animal tissues used as food. In some cultures "meat" refers only to muscles of mammals and therefore tissues of fish, birds or other animals are not considered as meat. Vegans support the broadest definition and do not consume any tissues of any animals in any form. That includes products derived from meat, such as meat stock, lard, gelatine, etc.
Vegans choose not to eat meat mostly because of ethical, environmental and/or health reasons.