Tissue refers to a group of cells that perform a similar function in an organism. It is the basic structural unit of living organisms and is essential for carrying out various biological processes. There are four main types of tissues in animals: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

Important points to remember about tissue include:

1. Composition: Tissues are composed of cells that are similar in structure and function. These cells are organized and bound together by intercellular substances, such as fibers and ground substance.

2. Function: Each type of tissue has a specific function in the body. Epithelial tissue covers and protects body surfaces, connective tissue provides support and connects different body parts, muscle tissue enables movement, and nervous tissue allows for communication and coordination.

3. Organization: Tissues are organized into higher levels of structural complexity. They combine to form organs, which then work together as organ systems to perform specific functions in the body.

4. Regeneration: Tissues have the ability to regenerate and repair themselves after injury or damage. This process is facilitated by the presence of stem cells, which can differentiate into different cell types and replace damaged cells.

5. Histology: The study of tissues is called histology. It involves the microscopic examination of tissue samples to understand their structure, composition, and function.

6. Diseases and disorders: Many diseases and disorders are associated with abnormalities or dysfunctions in specific tissues. Understanding tissue pathology is crucial for diagnosing and treating various medical conditions.

Overall, tissues are vital components of living organisms, playing a crucial role in maintaining the structure and function of different organs and systems.

A group of combined cells is called tissues which combine and form organs and they combine and form an organism.