What is direct heat?

I believe the direct v. indirect distinction originally comes from grilling. There, its essentially a question of do you put the food directly over the heat source (burning charcoal, gas burner, wood longs, etc.) or do you put it on the other side of the grill. Putting it directly over concentrates most of the heat on the bottom side of the food; putting it on the other side allows the heat to distribute to all sides of the food. Naturally, since its spread out, its also cooler.

So, you can then generalize that the following are more like direct heat:

• Sitting on the coals in a foil pouch. (Or on the wood, whatever fuel).
• Under a broiler, gas or electric
• Held with tongs over a burner on a stove (e.g., a pepper)
• In a sauté or fry pan, with only a little oil

The heat-contact side may be being hit by heat upwards of 1000°F in some methods.

And some examples of things more like indirect:

• Baked in an oven.
• In a smoker (what's called BBQ in the American South)
• Boiling, steaming, braising.

You also get some things that are harder to classify, like if you deep fry something its being cooked evenly all around, but at a heat delivery rate more similar to direct heat. And some odd things like a slice of bread in a toaster (mostly like direct heat, but with two heat sources).

Direct heat is used to cook thin cuts of meat (thin steak); indirect to cook large cuts (roast). Sometimes both are used; you may use indirect heat to cook a thick steak or roast through, but direct heat to sear the outside.

Direct vs. Indirect Heat are terms usually used when referring to American style BBQing or grilling. While the terms can apply to other areas of cooking as well their usefulness makes the most sense in this context.

Direct Heat

A method of heat transfer in which heat waves radiate from a source (for example, an open burner or grill) and travel directly to the item being heated with no conductor between heat source and food. Examples are grilling, broiling, barbecueing, and toasting. (source)

Keep in mind that the grill itself doesn't really act as a conductor as much as a way to hold the food in a static position relative to the heat source. To better understand direct heat, understand it's complement:

Indirect Heat

When using indirect heat, your goal is to never have any part of the meat directly over flames or charcoal. You can only do indirect cooking with a charcoal or gas grill that has a cover. In this respect, it acts very much like a convection oven... the heat swirls around the inside of the grill and the meat is cooked from all angles. This also eliminates the need to 'flip' or turn your meat during the cooking process. (source)

This is generally used to cook large pieces of heat that need a long time to cook. A smoker is an example of the kind of equipment used to cook with indirect heat.

I'm going to pull quotes from BBQ god Steven Raichlen to help answer your question:

Direct: food is cooked directly over and just a few inches away from the flowing coals at a temperature in excess of 500F.

Indirect: food is cooked adjacent to, not directly over, hot coals.

Direct heat is therefore hotter and fiercer, and suitable for thin, small tender cuts of meat (or breads), and will cook for a relatively short time. Therefore the images you're seeing are direct heat.

Indirect grilling is better suited for oven-style cooking: ribs, turkeys, whole fish.

In what context are you hearing these terms? I'm most familiar with direct vs indirect cooking with regards to grilling. In the context of grilling, direct heat means that the meat is directly over the flame, whereas indirect heat would be a configuration in which the coals are on one side of the grill and the meat on the other (often with a pan of water or other liquid underneath).

Indirect grilling is often used for slower cooking (ribs, brisket, etc) and provides more even heat. Direct grilling is great for quick searing, burgers, grilled pineapple, etc.

LOL this is getting silly - what are we going to say about induction hobs, rotisseries, and microwaves?

Perhaps we should think more deeply. Let me suggest that direct heat means heat directly applied to the food from a source that is red hot or hotter - flames are hotter than that by definition. That way electric heating is also accommodated.