According to linguistic research, a steady tendency towards the increase of compound terms has been observed over the last decades. Compound terms form a considerable layer in branch term systems or, more precisely, such nominations considerably exceed in number simple terms, because they make «over 70% of terms in different terminological systems».
Compound terms in various branch terminological systems of the Ukrainian language have been researched by: L. Kozak – in electrical terminology , S. Lokaychuk – in archaeological terminology , A. Chuyeshkova – in economic terminology , L. Kharchuk – in electric power engineering terminology , I. Protsyk – in physical activity terminology , N. Misnyck – in terminology of clinical medicine , O. Bohush – in astronomical terminology , P. Lunya – in terminology of constitutional law  and others. In biological terminology binary secondary word combinations were investigated . However, there is a need for a special analysis of the structural organization of biological terms, and compound terms in particular.
The aim of the article is to identify the peculiarities of forming compound terms of the biological term system, to determine the role of the word combination category as the mean of biological concepts nomination and to describe the most common structural models.
The biological term system of the Ukrainian language includes all structural types of terms: simple terms, composite terms and compound terms. It is clear that the basis of biological terminology is composed of one-component simple terms. In fact, at the beginning of forming any field, and biological in particular, simple terms play a crucial role.
The nomination of concepts by word combinations is especially typical for biological terminology in those cases when it is needed to set the place of a certain object in the row of other homogeneous objects, to distinguish it by pointing out special features.
Visible growth and extension of multi-component terms is caused by the necessity to reflect new discoveries, their innovative introduction in all spheres of our life. The development of biological science and the complication of scientific concepts created the need for their specification and thus make us introduce additional clarifications. For example, due to the invention of a microscope invisible for a human eye structures were discovered, a possibility of the detailed study of a cell appeared. The term ‘cell’ became the basis for creation some compound terms such as: polyploidy cell, haploid cell, segmental cell etc.
L. Symonenko stresses that in biological terminology “compound terms are widely used to systematize the plant and animal life terminology”. The presence of many motivating features (colour, shape, habitat etc) causes the introduction or an additional component to the simple term structure which makes this term more detailed. Therefore, compound terms are the means of scientific concepts specification.
Ukrainian biological terminological system includes a considerable number of compound terms. They perform not only a nominative function but also help to set the place of a denoted concept in the terminological system, representing the system ties of units and emphasizing differential features of a concept, i.e. facilitate the systematization of biological terminology. Compound terms reflect the development of science and testimony of terminological system building. The wide use of compound terms is explained by the desire to give the term that describes a new concept, greater semantic certainty.
Depending on the number of components, biological compound terms are divided into three structural types: two-, three-and multi-component. Most biological terms have a two-component structure. The most typical for the biological terminology is two-, and three-component terms. Quantitatively, the largest share among binary compound terms is represented by biological terminology model "adjective + noun", "noun + noun". As to the three-component terms there is a model: adjective + noun + noun; adjective + adjective + noun; noun + adjective + noun; noun + noun + noun. There is a small amount of multi-component terms in biological term system.
Compound terms form the integral part of biological terminology system, the existence of which is of an objective necessity. They reflect the entire integrated and structural complexity of the scientific field. Biological terms as the vocabulary layer, have prospects for future linguistic research, as this branch of science is characterized by a constant terminological development, creation of new and transformation of existing terminological units.