Define organ.

Explanation

**Step-by-step Solution:**

1. **Understanding Cells:** Start by recognizing that a cell is the basic structural and functional unit of any living organism. Cells are the building blocks of life.

2. **Definition of Tissue:** Cells group together to form tissues. A tissue is a collection of similar cells that work together to perform a specific function.

3. **Formation of Organs:** When different types of tissues come together, they form an organ. An organ is a structure made up of two or more types of tissues that work together to carry out a particular function.

...