Leave a steak alone in a hot pan and one face of it turns dark and crusted while the rest stays whatever color raw meat starts as. Toss diced onion in the same pan every few seconds instead, and every surface picks up a little heat, but none of it stays in contact long enough to do that. Searing holds food still against high heat until one patch of surface builds a real crust. Sautéing breaks that same contact into short, repeated bursts, cooking a piece through without crusting any one face of it.
Why holding still builds a crust and stirring prevents one
Browning a food's surface needs it dry and above roughly 300°F (150°C) for long enough that the Maillard reaction can run — that part is the same for every pan on the stove. What differs between searing and sautéing is how much of that time any single patch of the food actually gets. A steak sitting still in a hot pan keeps the same square inch of meat pressed against the same square inch of metal for the whole three or four minutes it takes to sear. A diced onion getting tossed every ten or fifteen seconds never holds that position long enough for one patch to climb past the browning threshold before it's moved somewhere cooler. It picks up color eventually, spread thin and even across the whole piece, but never the concentrated dark crust a still steak develops on one face.
Why "searing seals in the juices" doesn't hold up
The dark crust that forms during a sear is caramelized protein and sugar sitting on the meat's surface, not a shell sealing off the interior. Moisture still moves through it in both directions, the same way it moves through the pale surface of an unseared piece — searing doesn't slow how much the interior loses as it cooks. What the sear actually changes is flavor and texture on that thin outer layer, not what's happening underneath it. If a seared piece of meat tastes juicier at the table, the more likely reason is that it was pulled off the heat at the right internal temperature and rested afterward, not that the crust kept anything from escaping.
Piece size and fat point to different pans
Searing earns its place on pieces thick enough to have an inside worth keeping different from the outside — a steak, a chop, a chicken thigh, a roast headed for the oven. A single seared face develops a crust while the center stays rare, medium, or however the piece is meant to finish. Cut that same meat into strips or cubes and searing stops making sense: there's too much surface and not enough interior for a still crust to mean anything, and turning the pieces to brown every side would take a still pan far longer than a moving one.
Fat plays a different role in each. A sear usually gets by on a thin film of oil, just enough to keep the meat from welding itself to the pan, because the food itself is doing most of the work of holding heat against its own surface. A sauté leans on more fat relative to the food, since it's the fat carrying heat into pieces that are only ever touching the pan's surface for a moment at a time — coating them in hot oil or butter between contacts keeps them cooking evenly instead of drying out where the pan touches and staying raw everywhere else.
How to tell each one is working
A sear announces itself with a hard, steady hiss the moment food hits the pan, loud enough to talk over, and it should keep sizzling hard for the first thirty seconds or so rather than going quiet. Lift the edge with tongs after two or three minutes and it should have gone deep brown to nearly black in patches, not tan — a pale, greasy-looking surface after that long means the pan wasn't hot enough or there was too much food crowded into it, dropping the temperature and steaming the meat instead of browning it. The smell turns toasted and slightly sharp, almost like roasted nuts, well before the crust looks finished.
A sauté sounds lighter and more constant, a steady crackle rather than one loud hiss, because the food is never in one place long enough to smother the sound with its own released moisture. Color builds slowly and evenly across cut edges and corners first, going from raw to gold rather than raw to brown in one jump. Onion and garlic sautéed properly smell sweet and mellow rather than sharp, and a piece that's ready offers little resistance against the side of a spoon or spatula.
Why a seared piece of chicken still needs a thermometer
None of this is a safety concern for a piece of meat that's fully cooked by the time it leaves the pan — a thin cutlet seared through on both sides, or a piece finished the rest of the way in the oven after a hard sear. It becomes one when a thick piece is only seared, not finished, and the color on the outside gets mistaken for doneness on the inside. Chicken has to reach 165°F (74°C) at the center to be safe, and a two-minute sear on a thick thigh can produce a dark, appetizing crust while the middle is still well short of that. Beef, pork, and lamb allow more judgment by color and feel once they're cooked through, but chicken doesn't — a thermometer is the only way to know a seared piece is actually done, not just browned.