A heterogram (from hetero-, meaning 'different', + -gram, meaning 'written') is a word, phrase, or sentence in which no letter of the alphabet occurs more than once. The terms isogram and nonpattern word have also been used to mean the same thing.[1][2][3]
It is not clear who coined or popularized the term "heterogram". The concept appears in Dmitri Borgmann's 1965 book Language on Vacation: An Olio of Orthographical Oddities but he uses the term isogram.[4] In a 1985 article, Borgmann claims to have "launched" the term isogram then.[5] He also suggests an alternative term, asogram, to avoid confusion with lines of constant value such as contour lines, but uses isogram in the article itself.
Isogram has also been used to mean a string where each letter present is used the same number of times.[6][2][7] Multiple terms have been used to describe words where each letter used appears a certain number of times. For example, a word where every featured letter appears twice, like "Shanghaiings", might be called a pair isogram,[8] a second-order isogram,[2] or a 2-isogram.[3]
A perfect pangram is an example of a heterogram, with the added restriction that it uses all the letters of the alphabet.