Trenchcoat

A trenchcoat, or trench coat', is a raincoat made of waterproof, heavy-duty cotton gabardine drill or poplin, or leather. It generally has removable insulated lining, ragline sleeves, and the classic versions come from above the knee to meeting at the mid-calves. The garment traditionally is double-breasted with ten buttons, has wide lapels, a storm flap, and pockets that close.

Usage
They were initially used during World War I by Britain and France as a replacement to the heavy siege greatcoats in the same war.

Big Boss, during his command of FOXHOUND and Outer Heaven, as well as when reuniting with Solid Snake in 2014, wore a biege trenchcoat.

Gene, when leading FOX, wore a trenchcoat.

The KGB Agent Vladimir Aleksandrovich Zadornov, while posing as the UN University for Peace Professor Ramon Galvez Mena, wore a trenchcoat when meeting Big Boss, as well as when imprisoned on Mother Base up to his death.

A sunglasses-wearing member of Diamond Dogs wore a trenchcoat during 1984.

Multiple members of FOXHOUND during 2005 have been seen wearing long trenchcoats with seven buttons and padded shoulders, including every member of the Sons of Big Boss.

Liquid Ocelot wore a trenchcoat with his business suit during his insurrection against the Patriots.

Behind the scenes
Depending on the game/concept art, the color of the coat tends to change, but as of Metal Gear Solid 4 and the Liquid Snake Play Arts KAI action figure, the official color is tan.

Roy Campbell in the non-canon Metal Gear Solid radio drama wore a trenchcoat in his artwork.