**1. Introduction**

The use of plant and plant-derived natural products for medicinal, religious, and cosmetic purposes has a history dating back to the emergence of humanity. Exploring natural plant products as an option to find new chemical entities as leads is one of the fastest growing areas of research. Medicinal plants are rich sources of bioactive phytochemicals and/or bionutrients, which have shown important role in preventing chronic diseases like cancers, diabetes, and coronary heart diseases [1]. It is well documented that plants produce these chemicals to protect themselves, but they also protect plants from diseases and damages and contribute to the plant's color, aroma, and flavor [2]. The pharmaceutical properties of aromatic plants are partially attributed to essential oils (EOs), which can also be seen as an important group of plant secondary metabolites. Although the use of EOs has been primarily related to food flavorings, cosmetics, and perfumes due to their aroma, research demonstrates the high potential of the use of volatile monoterpene constituents to cure and prevent human diseases [3, 4]. During the recent years, plant EOs have come more into the focus of phytomedicine and aromatherapy; hence their widespread use has raised more interest to scientists in basic research, especially their antimicrobial, antioxidant, and anticancer activities. In general, EOs consist of chemical mixtures involving from several tens to hundreds of different types of molecules, most of them being complex natural mixture of terpene and

phenylpropanoids (benzene derivatives) which are responsible for their biological activities [5, 6]. At the first glance, terpenes and EOs can seem alike; both can come from plants and are aromatic; for many they are used for the same purpose. These similarities have led to a wide misconception that they are same, but this is not necessary the case [7].
