Two vectors are orthogonal if the addition of one of them does not affect the "amount" of the other, when using them as basis vectors. In this case, "(1,0)" and "(0,1)" are orthogonal; no matter how far in X you go, the amount of Y-ness doesn't change. (1,1) and (1,-1) are also orthogonal. In the metrics most people learn, this means the vectors are at right angles. (Forgive me this simplification, mathematicians, going further will only obscure things.)

Programmers tend to abuse this term to mean "the two things are totally unrelated", even though the term really only applies to certain mathematical constructs like vectors. Claiming that type-safety is orthogonal to garbage collection would mean the two are entirely separate concerns and that you can freely have none, one, or both, and that the two issues never matter to each other. Now, technically this isn't quite true, which is why I say we "abuse" the term, because the full meaning of "orthogonal" is that you will literally never be considering the effect of one while examining the other, and this is not true; type systems do some impact on GC, for instance. However, in many cases this is "true enough" for someone to say they are "orthogonal".

In this case, I'm not arguing merely that the precise definition is not met, because it pretty much never is, but that the spirit in which programmers use the term is also not applicable here, as the questions of "code quality", "speed", and "developer experience" are indeed tightly interrelated and so answers like "you always have to sacrifice quality for speed" are too simplified.

In simple English, orthogonal means "at right angles". People often say that two things are orthogonal when they mean is that they are different dimensions that are unconnected with each other.

To pick a silly example, the number of coffee cups I have is orthogonal to the number of jars of jelly beans I have.