Hands of God and Adam

Ceiling of Sistine Chapel, Michelangelo's Creation of Man.

What are monotheism and monotheistic religions? Well, the best way to explain the monotheistic definition is to compare it to the opposite idea: Polytheism.
So therefore, what is monotheism and polytheism?

Monotheism vs Polytheism
What is monotheism and polytheism, and what is the difference between monotheism and polytheism?
Basically a monotheist believe that there is only one God, whereas a polytheist believes that there are more than one. In a polytheistic religion there could be two Gods, such as one male and one female, or one Good God and one Evil God; or there could be many Gods, each representing different aspects of life and death.

Another aspect of religion is pantheism, which is related to the question: Where is God?, The key question is whether God is transcendent i.e. above and outside the world? Or whether God is immanent i.e. to be found in the world, meaning everywhere, all over the universe.
A pantheist believes that God, the Force, Pan, the Great Spirit is everywhere. That God or the Gods exist in all living creatures, in the earth, water, and air, and in each stone and all other material things, etc.

What religions are monotheistic?
First a further comment about pantheism. If a religion define God as the supreme being, who is immanent in nature and/or all other things, then it can be debated whether such a religion is monotheistic or polytheistic.
An example of this dilemma is Hinduism that has numerous Gods and Deities, and therefore it is common to regard Hinduism as a polytheistic religion. However, the Hindus believe that there is one supreme God, who doesn't self have any form or characteristics, but is represented in all other holy beings. It is therefore understandable that it can be an endless debate over whether Hinduism is based upon monotheism or polytheism.
It is commonly agreed upon that Hinduism isn't regarded as monotheistic, and that the three main monotheistic religions are Judaism, Christianity, and Islam.

Judaism, Christianity, and Islam
These three religions have the same fundamental belief that God is the Creator of all things. And it must be emphasized that it is the same God; and all three religions tell (more or less) the same story of the creation of the world and the creation of mankind. That man is created by God is perfectly illustrated in Michelangelo's painting on the ceiling in the Sistine Chapel, With the focal point of the Hands of God and Adam, showing the exact moment when God gave life to Adam as the representative of all mankind
The three holy scripts follow the same outline of the holy tales until the story about Abraham and his sons; at this point the believers belonging to Judaism and Christianity continue to follow the same path. Whereas Islam breaks away from the other two and follow in their historical script, the Koran, an illegitimate son of Abraham.
The believers of Judaism and Christianity follow the same scripts until the birth of Jesus, and it has to be emphasized that many of the holy scripts of Judaism are part of the Christian's Bible as the Old Testament.
It is a big mystery why these three religions for more than a thousand years have fought against each other. All believe in the same God, who to the followers of all three religions speak about peace and love.

It is without any doubt correct to say that Judaism is one of the monotheistic religions. That there is only one God is made very clear by the Ten Commandments brought by Moses. And from the viewpoint of Judaism, then it is understandable that the Jews became very upset when Jesus arrived on the arena and claimed to be the Son of God.

Islam is also very clearly a monotheistic religion. One of the five Important fundamental pillars of Islam is: There is only one God!

The question, whether Christianity is a monotheistic religion is a little more blurred, but one key issue is one of the Ten Commandments You shall have no other gods before me. But later it becomes more difficult to comprehend; How can God, Jesus, and The Holy Spirit all three be God? How is this Trinity God to be comprehended. Furthermore, the Catholics worship The Virgin Mary and many other saints.
However, having made these comments, then the conclusion which almost all agree upon is that Christianity is a monotheistic religion.

Ref: Ceiling of Sistine Chapel, Michelangelo's Creation of Man