English term for a word that differs from another one by just one letter

Solution 1:

I was able to find the answer myself, thanks to help from Chris.

The answer to the first question is “orthographic neighbor”, introduced by Coltheart, Davelaar, Jonasson and Besner in 1977, or “substitution neighbor” in more recent research. The answer to the second question is “addition/deletion neighbor”.

Googling for these terms returns hundreds of relevant results, mostly papers such as:

• Previewing the Neighborhood: The Role of Orthographic Neighbors as Parafoveal Previews in Reading (PDF)
• Children Like Dense Neighborhoods: Orthographic Neighborhood Density Effects in Novel Readers (PDF)
• Phonographic neighbors, not orthographic neighbors, determine word naming latencies (PDF)

Some great reading there.

Solution 2:

For your first question: You could use the term "minimal pair".

As the wikipedia article on this term correctly states, its main use is in phonology. Nevertheless, since the underlying principle is the same, it should be okay to use for written words, too. I have also seen it put to use for whole sentences, where the difference is "a word at some position".

Solution 3:

I've see pairs of words that differ by 1 letter referred to as a word chain. A technical term for them seems to be orthographic pairs.

Solution 4:

Lewis Carroll apparently originated the word ladders game and called it "doublets."