The anthropological approach of studying human societies as integrated wholes consider religion as a part of culture. Anthropologists try to find out the relevance of religion in human societies, whether primitive or technologically advanced, and the significance of religion in human societies. Notable is the fact that there is no society known so far without any religious idea, it is a cultural universal. They made attempts to search for earlier forms of religion and religious thoughts and the courses of change therein. Some intellectuals thought that religion will have no place where science and technology flourish - but the reality is to the contrary.
Anthropologists defined religion in different ways, but none of them adequately cover all aspects of religion practised by all human societies.
Edward Burnett Tylor : "Belief in spiritual things"
Emile Durkheim : …"a unified system of beliefs and practices relative to sacred things, that is to say, things set apart and forbidden -- beliefs and practices which unite into one single moral community called a Church all those who adhere to them."
Clifford Geertz : "A religion is a system of symbols which acts to establish powerful, pervasive, and long-lasting moods in men by formulating conceptions of a general order of existence and clothing those conceptions with such an aura of factuality that the moods and motivations seem uniquely realistic."
for more - Various Definitions of Religion
Typical dictionary definition of religion : "belief in, or the worship of, God or Gods."
References: IGNOU Study Materials