umbkeelsed
Umbkeelsed is a term used in Estonian linguistics to describe a specific grammatical phenomenon related to verb conjugation. It refers to verbs that exhibit a particular pattern of sound change or assimilation in their stems when they are conjugated. This change often occurs in certain grammatical forms, such as the past tense or participle forms. The concept of umbkeelsed is not about a separate class of verbs in the same way as, for example, weak or strong verbs in Germanic languages, but rather a descriptive label for a set of observed phonetic and morphological alterations within the conjugation of various verbs.
The term itself is derived from Estonian words that roughly translate to "hidden" or "obscured" and "verbs."