Polymethanal is a common fixative in biology. It is used to reduce degradation in cells, tissue and entire organisms before further experiments, e.g. antibody staining.
Too many names
There's plenty of confusion regarding this chemical because too many synonyms exist. Paraformaldyhde is probably still the most common term among biologist, but it ignores attempts to introduce meaningful and systematic names in chemistry. Polymethanal is clearer: poly - polymer, meth - single carbon, -al - aldehyde group.
Comparison with methanal
Polymethanal is often preferred over methanal which is also used in fixation of live material. This is because polymethanal is a solid which makes it easier to transport and a little safer to use. The chemistry of fixation is similar due to the partial depolymerisation of polymethanal in the buffer making process (basic hydrolysis).