Uczestniczk
Uczestniczk is not a standalone word in standard Polish. It is the stem used in the inflection of the feminine agent noun derived from the verb uczestniczyć (to participate). The common feminine noun is uczestniczka, meaning a female participant. The corresponding masculine noun is uczestnik (a male participant). The terms are related etymologically, but they belong to different nominal formations.
In grammar, the stem uczestniczk- appears across the inflection of uczestniczka: singular nominative uczestniczka; genitive uczestniczki;
Usage notes: In everyday language, one typically uses uczestniczka or uczestnik rather than referencing the stem
See also: uczestnik, uczestniczka, uczestniczyć, uczestnictwo.