Witcher's Steel Blade is one of the steel swords that Geralt receives at the beginning of the game, during the Prologue (the very first being a rusty sword). Has the general two-handed longsword model.
Although it is used primarily against humans and reportedly has little effect on monsters, Geralt is able to fight off most monsters during the early stages of the game using only this sword.
In the Prologue, Vesemir mentions that witchers have good cold steel. The best improvement for this sword is to have a meteorite sword forged.
Witcher's steel sword is the default steel sword which is equipped by Geralt when starting the game. It is part of the starting inventory which can be accessed from the beginning of the quest Lilac and Gooseberries.