Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. The germ layers form during gastrulation of the blastula. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers.
All "higher" and "intermediate animals" (from flat worms to humans), arise from a triploblastic ovum. Triploblastic organisms are organisms that develop from such an ovum. Generally speaking, triploblastic organisms belong to the Bilateria subregnum. Triploblastic organisms generally possess bilateral symmetry, which is where the clade Bilateria takes its name.
Simpler animals qualified as diploblastic, such as cnidaria (which includes jellyfish, corals and hydra), possess two germ layers. Even simpler animals, such as sponges within the formally termed Porifera phylum, contain no true tissues.