In three-dimensional space, there are three primary spatial directions: left/right, forward/backward, and up/down. These directions are typically referred to as the X, Y, and Z axes, respectively.
When we talk about directions in three-dimensional space, we usually consider them in relation to a coordinate system. In the commonly used Cartesian coordinate system, the X-axis typically represents the left/right direction, the Y-axis represents the forward/backward direction, and the Z-axis represents the up/down direction.
Regarding your question about up/down from Earth's surface, it's important to note that the concept of up and down can be relative depending on the reference frame or context. Relative to the Earth's surface, we generally consider "up" as the direction opposite to gravity, pointing away from the Earth's center, and "down" as the direction aligned with gravity, pointing towards the Earth's center.
However, when we discuss three-dimensional space in a broader context, such as in physics or mathematics, the terms "up" and "down" become less meaningful because they depend on the specific reference frame being used. In such cases, we typically refer to the vertical direction as the Z-axis or use other terms to describe directions in a more neutral manner.
In summary, in the context of a three-dimensional Cartesian coordinate system, the directions include left/right, forward/backward, and up/down. However, when discussing directions in a broader context or outside Earth's reference frame, the terms "up" and "down" can become more relative and less applicable.