Bigram is a coordinate term of trigram.

In linguistics terms the difference between trigram and bigram

is that trigram is a special case of the n-gram where n is 3, used in natural language processing for doing statistical analysis of texts while bigram is a pair, often of words or tags, used in analysis.

As nouns the difference between trigram and bigram

is that trigram is in Chinese Taoism, a symbol composed of three marks each of two types (creating 8 possible symbols) which are used for philosophical discussion and divination (fortunetelling) while bigram is a pair, often of words or tags, used in analysis.