single word requests - Name for a special type of abbreviation, such as "mart" for "market"

"Mart" is a word coined in the middle of the 1400s. The abbreviation for "market" is "mkt". If it was borrowed from Middle Dutch "markt", the orthographic shortening is understandable: English phonetics doesn't provide for the sound represented by kt in Dutch.

It seems to me to be merely a respelling, just as American English has changed British English spellings of words like "foetus" to "fetus", "haematology" to "hematology", and "colour" to "color", and, for example, the German spelling of München to Munich.