Spatially we consider our universe to exist in 3-dimensions of horizontals and verticals with the 4th-dimension of space being space that is outside of and surrounding the other three dimensions and at angles. For example, the universe may be said to be the 4th-dimension. Being outside of and around us on the 4th-dimensional plane, and because of our 3-dimensional existence, we cannot perceive or even conceive of the universe’s size and or shape.

Time is considered to be one dimension although we conceive of it in a linear three dimensional way of past, present and future. Our minds conceive of them as separate units hence three dimensions in spite of their continuous uninterrupted flow. And our ability to conceive of and understand that those three linear dimensions are in truth a singular one, divided and defined and segregated in our minds by relativity, being the fourth. Simply, there is no such thing as past, present or future; there is only now.

4s + 1t = 5d

By this equation, we see that the fifth dimension is the sum of both space and time. Carl Sagan discusses it here:

If the 5th dimension is the sum of both space and time, what is the 6th dimension?