What term describes a vast number or countless?

The term that describes a vast number or countless is "myriad." It originates from the Greek word "myriad," which means ten thousand, but in modern usage, it has come to mean an exceedingly great or indefinitely large number of things. This versatility in meaning makes it particularly useful when discussing quantities that are not easily specified, emphasizing abundance without limitation.

In contrast, the other choices do not convey this meaning. "Nonchalant" refers to a casual or indifferent attitude, which has no relation to quantity. "Pique" is associated with arousing interest or curiosity, and "outmoded" signifies something that is outdated or no longer in style. Therefore, "myriad" stands alone as the term that accurately encapsulates the idea of a vast or countless number.