This term refers to a word that is the ancestor of a word in a modern language. Proto-words are often reconstructed from the study of related languages, and they are typically written in a special script or notation.
For example, the proto-word for "father" in the Indo-European language family is reconstructed as *pater. This word is the ancestor of English "father," German "Vater," Latin "pater," and many other words in related languages.
It's important to note that proto-words are not actual words that were spoken, but rather reconstructions based on linguistic evidence. They are hypothetical forms that represent the likely ancestor of a word in a language family.