Agism is the American spelling of ageism: treating people unfairly because of how old they are — most often by pushing older people aside, though it can also hit the young. The idea is the same kind of unfair -ism as racism or sexism, only the target is age. Most British books, and most Taiwan textbooks, keep the e: ageism. American papers sometimes drop it: agism. Neither spelling changes the meaning. Though actually, you should recognise both, then write the one your teacher or dictionary asks for — in many classes that will be ageism.