The mathematical term for dropping letters is taking a "subsequence", https://en.wikipedia.org/wiki/Subsequence https://en.wikipedia.org/wiki/Subsequence . This

The mathematical term for dropping letters is taking a "subsequence", https://en.wikipedia.org/wiki/Subsequence https://en.wikipedia.org/wiki/Subsequence . This is not the same as taking a substring, which is understood to be a contiguous subsequence.