Koostisosa

Koostisosa is a linguistic term used in Estonian to describe a component of a compound word. A compound is formed when two or more morphemes or words are combined to create a new lexical item, and each part of the combination is referred to as a koostisosa. The concept helps describe how the meaning and form of the whole word arise from its parts.

In most Estonian compounds, the rightmost constituent functions as the syntactic head, with the other constituents acting as modifiers. The head often determines the grammatical behavior of the whole word, including its inflection. For example in koolibuss (school bus), buss is the head and kooli modifies it; the compound typically inherits the inflectional properties of the head.

Koostisosa can include various types of constituents. They are not limited to full words; some compounds involve bound morphemes as part of one element, while others are composed of two or more independent words. The relationship between constituents can be purely lexical-semantic (kooli = school as a modifier of bus) or more opaque, depending on historical language change.

Compounds can be endocentric, where the whole refers to something of the same kind as the head, or, less commonly in Estonian, exocentric, where the meaning cannot be straightforwardly predicted from the head alone. Depending on formation and usage, compounds may be written as closed words or with connecting elements in certain stylistic contexts.

Koostisosa thus denotes the building blocks of compound words, helping to analyze how new words are formed and how they behave in Estonian grammar. Examples include koolibuss (school bus) and raudteejaam (railway station). See also Estonian morphology and compound word formation for related topics.