What does perpendicular mean in earth science

Two lines that form a T are perpendicular to each other. They meet at a right angle. A person who is standing is perpendicular to the earth. Use perpendicular to describe lines, angles, and direction. In geometry a perpendicular angle is 90 degrees, a perfect L.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.