Why is the first world war called "the first world war"?Explain with reason.

To answer this, you have to look at both parts of the
name.

First, this was a world war because it involved
countries in practically every part of the world.  European countries were most strongly
involved, but there were American countries, Asian countries, and even countries such as
Australia and New Zealand.  African countries were affected because their colonizers
took many of their men to work or fight in the war.  In this sense, it was a world
war.

In addition, this was the first of the two world
wars.  That's why it is called the "First" World War.  Please note that this name was
not applied to it until the Second World War came around.  Before that, it was known as
the Great War.