Half-Fiends are a type of creature in Dungeons & Dragons who, as the name suggests, are the result of interbreeding between other races and fiends. Alongside Half-Dragons, they are perhaps the most iconic of the "mixed race templates".
In Advanced Dungeons & Dragons, half-fiends did not exist; instead, there were two distinct monsters reflecting fiendish interbreeding, in the form of the Cambion and the Alu-Fiend. The Half-Fiend template came into existence in 3rd edition.
Half-Fiends are the direct progenitors of tieflings.