N-gram

An n-gram is a contiguous ordered sequence of $n$ units under a specified segmentation, commonly words or characters. A one-unit sequence is a unigram and a two-unit sequence a bigram. In a text corpus, counting n-grams retains local ordering that individual-word totals discard.^[ref]

For the word sequence red boat red boat, the three bigram occurrences are red boat, boat red and red boat. Thus red boat accounts for $2/3$ of these bigram windows, although the same sequence occurs twice. For $L$ units and $1\le n\le L$, the number of unpadded windows is $L-n+1$.

The units and denominator are part of the definition of a measured rate. Character bigrams differ from word bigrams, and word segmentation, capitalization and sentence boundaries can change the count. A reproducible analysis states those choices before comparing time series or language varieties.