From the Greek κύστις "bubble".

Cyst - a dense shell in many parasites and other invertebrates, used to maintain vitality in unfavourable environmental conditions. Some single-celled organisms can exist in the form of a cyst for up to several years under unfavourable conditions.

In protozoa (some flagellates, infusoria, etc.) 2 types of cysts are formed - dormant cyst (for the period of unfavourable external conditions) and reproductive cyst (for the period of division). In plants (algae, etc.), bacteria, a dormant cyst is formed, which leaves the shell after the end of the unfavourable period.