By blow, also commonly spelled as byblow, is a term that refers to an illegitimate or secret child born out of wedlock. While this term may have been commonly used in the past, it is considered outdated and potentially offensive in modern times. There are several synonyms for the term by blow that can be used instead, including love child, illegitimate child, bastard, secret child, and out of wedlock child. Using more modern and appropriate language can help to avoid offending individuals while still conveying the same message or meaning.