These facts then clearly evidence such elements as enclitical particles which exercise certain morphological and/or syntactical functions, but which have not yet developed into actual suffixes obliged to conform to sound-harmony and the general accentuation pattern. Some elements of this type are the formantia of the cas. compar. in -däg, and the cas. aequat. (prosecut., mensurat., terminat.) in -ča/-čä in the nominal category, and the formans of the negative aspect in -ma-/-mä- in the verbal category.
Source: wiktionary
In Cowgill’s […] convincing opinion the basic formans of this PIE mediopassive was an -o(-) originally added to endings identical with those of the perfect (minus -e where applicable;[…]), which only had a single set of endings undifferentiated as to active/middle in PIE.
Source: wiktionary
After the selection of an onomasiological base and an onomasiological mark on the semantic level of the word-formation process, the speaker selects a word-formation base and a formans from an inventory of productive word-formation categories, classes, and subtypes on the formal level.
Source: wiktionary