- a spelling different from the one in current use
- use of the same letter(s) to convey different sounds, for example, gh in rough and ghost.
Earlier, when English was mainly a spoken language, it was a free-for-all, spelling-wise. Any spelling was good as long as you could make yourself understood. Each writer spelled words in his own way, trying to spell them phonetically. Shakespeare spelled his own name in various ways (Shaxspear, Shakespear, and so on).
If you read old manuscripts, you can find different spellings of a word on the same page, and sometimes even in the same sentence. Spelling wasn't something sacrosanct: if a line was too long to fit, a typesetter might simply squeeze or expand the word by altering the spelling.
If the idea of to-each-one's-own spelling for the same word sounds bizarre, consider how we practice it even today, in the only place we can: in our names. Look around you and you might find a Christina and a Cristina and a Kristina and many other permutations and combinations.
With the advent of printing in the 15th century, spelling began to become standardized. By the 19th century, most words had a single "official" spelling, as a consensus, not by the diktat of a committee.
Today if you write "definately" and someone points out that you've misspelled the word, just tell them you're a practitioner of heterography.