© Randal Oulton
Preheat means to heat before cooking.
The purpose is to get a pan or oven to a specified temperature before you use it to cook with.
Many North American recipes are starting to show their age, in that they advise you right off the bat to “preheat your oven”, and then commence an hour or more of prep work for the actual dish. Such recipes were obviously written at a time when the cost of energy was thought of as “free” or so little as to not be worth thinking about.
If you know your oven only takes 10 minutes or whatever to come to heat, and come across a recipe instruction that would have a fully-heated oven sitting unnecessarily idle for a longer period of time, disregard the instruction to start heating right at the beginning and start your oven heating instead at the right time point for your oven.
That aside, Preheat is such bad English, but it’s too late to do anything about it. Just as bad money drives out good money, bad English drives out good English. Even some crock pot recipes will call for you to preheat a crockpot. What people really mean is just plain old “heat”; no “pre” is needed on the word.