A significant drop in the share price of many/most listed companies at the same time, typically over a short duration (weeks to months).
Notable recent examples of this were as Covid19 hit and before that the GFC.
Here is a weekly chart of the XJO (index that roughly tracks the Australian market), the Covid19 correction started at the red arrow and ended at the green one:
It is my personal view we are due for another one soon and it will be larger than the one in 2020. Others may disagree.
