The word "spike" can have several meanings. Let's consider two common ones and find their antonyms.
Meaning 1: A sharp point or projection.
Antonyms for this sense of "spike" could be "blunt" or "rounded". When something is spiked, it has a sharp, pointed end that can pierce or stick into things. In contrast, something that is blunt has a dull or rounded end and lacks the sharpness of a spike. For example, a nail with a spike on the end is very different from a nail with a blunt tip. The spiked nail can easily penetrate wood, while the blunt nail would have a hard time doing so.
Meaning 2: A sudden, sharp increase.
Antonyms for this meaning could be "decline", "decrease", or "drop". When there is a spike in something like prices, numbers, or activity, it means there is a sudden and significant upward movement. On the other hand, a decline, decrease, or drop indicates a downward movement. For instance, if there is a spike in the stock market, prices go up rapidly. But if there is a decline in the stock market, prices fall.
In summary, depending on the meaning of "spike", different words can serve as antonyms. If it refers to a sharp point, words like "blunt" or "rounded" are antonyms. If it refers to a sudden increase, words such as "decline", "decrease", or "drop" are the opposites.