In 1866, Ernst Haeckel coined the term 'phylum' for the classification of animals. It is the category below the Kingdom and is made up of various classes sharing common features. The same category is called the 'division' for plant classification and this term was given by August W. Eichler in 1883.

Introduction:
The term phylum refers to the highest taxonomic category in the animal kingdom. It is a rank above class and below kingdom. The concept of phylum was first introduced by a German biologist, Ernst Haeckel in the mid-19th century.

Explanation:
Ernst Haeckel was a prominent zoologist who was interested in the classification of organisms. In his book "Generelle Morphologie der Organismen" published in 1866, he proposed a new system of classification based on the concept of phylum.

Haeckel believed that all animals could be grouped into a limited number of major groups, which he called phyla. Each phylum was defined by a set of unique characteristics that distinguished it from all other phyla.

For example, animals belonging to the phylum Arthropoda have jointed appendages and an exoskeleton made of chitin, while animals belonging to the phylum Chordata have a dorsal nerve cord and a notochord.

The concept of phylum was later adopted by other biologists and is now widely used in the classification of animals. Today, there are over 30 recognized animal phyla, ranging from the simple sponges to the complex vertebrates.

Conclusion:
In conclusion, the term phylum was first introduced by Ernst Haeckel in the mid-19th century. He proposed a new system of classification based on the concept of phylum, which grouped animals into a limited number of major groups defined by unique characteristics. This concept has since been widely adopted and is now an important part of the classification of animals.