Although there’s no strict definition or set of guidelines for this eating practice, “a pescatarian is someone who includes fish while maintaining a vegetarian diet. In other words, a pescatarian is a person who will consume fish and seafood, but not red meat, chicken, or pork,” explains Amy Kimberlain, RDN, CDCES. “They will also include tofu, beans, vegetables, fruits, dairy, and grains. Some vegetarians eat eggs and dairy, some do not.” 

At a glance, pescatarians and vegetarians might look very much alike, with a shared focus of plants as their foundation. But their chief distinction lies in their source of animal (fish and seafood) vs. vegetable protein. A vegetarian does not eat fish, and a pescatarian does. 

It’s also important to note that pescetarianism isn’t new to human history. Many civilizations have followed a fish-dominant, plant-based diet for religious, social, health, and cultural reasons throughout millennia. However, the term pescetarianism—and the eating behaviors associated with it—didn’t officially appear in mainstream culture until the late 20th century. 

https://www.mindbodygreen.com/articles/what-is-pescetarianism-health-benefits-of-the-diet-what-to-eat-and-recipes