Introduction : Euclid was a Greek mathematician, often referred to as the "Father of Geometry," who lived around 300 BCE. While not much is known about his personal life, his contributions to mathematics have had a profound and lasting impact. 

Early Life: Euclid's birthplace and exact birth and death dates are not known, but it is believed that he lived around 300 BCE in Alexandria, Egypt. Some historical accounts suggest that he may have studied under the students of Plato in Athens. Euclid's work primarily focused on geometry, but he also made contributions to other areas of mathematics.

Main Contributions: Euclid's most famous work is "Elements," a comprehensive compilation of mathematical knowledge of his time. "Elements" is a series of 13 books that cover a wide range of mathematical topics, including geometry, number theory, and mathematical logic. The structure of "Elements" laid the foundation for the way mathematics is organized and taught.

Some of his main contributions include: 

Axiomatic System - "Elements" is based on a set of axioms and postulates from which the entire body of geometric knowledge is derived.

Euclidean Geometry: Euclid's work in geometry, particularly in plane geometry, has been highly influential.

Number Theory: Euclid's "Elements" includes a book on number theory, where he explores the properties of prime numbers and provides a method to find the greatest common divisor of two numbers (Euclidean algorithm).

Mathematical Rigor: Euclid emphasized the importance of rigorous proofs in mathematics. His deductive approach to reasoning and geometric proofs set a standard for mathematical rigor that has endured for centuries.

Key Definitions with Pictures:

Point: A point is that which has no part. (Imagine a tiny dot, so small it has no measurable width or length.)

Line: A line is a length without breadth. (Think of a thin, infinitely long thread stretched straight.)

Angle : The inclination of two lines to each other, meeting at a point. (Picture two lines coming together at a corner, forming an angle.)

Triangle: A plane figure bounded by three straight lines, and containing three angles. (The most basic three-sided shape.)

Circle : A plane figure bounded by a curved line, every point of which is at the same distance from a fixed point called the center. (A perfect, endless loop.)

Death and Legacy:

Euclid's contributions extend far beyond the realm of mathematics. His work on geometry has had profound implications for architecture, engineering, art, and even philosophy. His legacy serves as a testament to the power of human thought and the enduring value of intellectual pursuit.