A terrine is actually a dish, as in the pan used to make this dish, but slowly the word is being extended to describe the food cooked in it as well.
The dish has straight sides, is oblong and is usually made of earthenware.
Really, what’s made in it is a version of pâté, but because it’s made in a terrine, it’s a terrine (though if you threw a meatloaf in the same pan and tried to call that a Terrine too, the foodies might object,)
A terrine is usually made of meat. The cooking technique allowed appealing dishes to be made from cheaper cuts of meat, reformed and seasoned. A seal of lard on top of the dish, allowed it to be stored in a cool place, if only for a few weeks, without spoiling. It was classic country peasant food. It was only adapted in the last 1700s / early 1800s by the food trade in Paris to feed people there. In Paris, they developed gourmet forms of it, using goose livers and truffles, and decorated with shaped food items.
Ingredients can be layered and put in the terrine in such a way that when you cut it, each slice reveals a pattern — a classic trick is square batons of foie gras.
A Terrine needs to be cooked slowly in an oven, so the dish is usually set in a water bath. The meat mixture cooks without browning, and its juices form pockets of jelly.
After it’s cooked, the Terrine needs to be pressed to get rid of any air pockets. Air pockets would be ruin the texture, and possibly make slices crumble. (As well, with an eye to preservation in the days before refrigeration, air pockets would shorten its shelf life by acting as breeding patches for some bacteria.) The pressing starts once the Terrine has cooled (pressing it warm would force the juices out.)
Terrines are served cold or room temperature. They typically have lots of salt in them — this both helped with their shelf life, and made them taste less bland than they would otherwise cold.
They are often served with side items whose taste can cut through and compliment the fat taste of the Terrine — items such as chutneys, caperberries, cornichons, and fruit preserves.
“Terrine” comes from the French word “terre”. The dishes were traditionally earthenware.