An "ism" is an idea or movement that people believe in strongly. For example, feminism is an important social and political ism.
When we talk about an "ism", it usually means a way of thinking or acting that's based on a particular set of principles or ideas. Think about all the different types of music – jazz ism, rock ism, folk ism... Each one has its own unique style and attitude, but they're all connected by their shared spirit. In the same way, social movements like environmentalism or socialism are called isms because they represent a particular set of values and goals that people work towards.
The suffix "-ism" can also be added to other words to create a new word with a slightly different meaning. For example, nationalism means loving your own country strongly, but communism (or any other type of "ism") refers specifically to an economic system where everything is owned by the community as a whole.
An ideology, system of thought, or practice that can be described by a word ending in -ism.
