## 2 Terpenoids  
Terpenoids, also known as terpenes, are the largest group of phytochemicals, and are present in many different plants, playing an array of biological and biochemical roles within living systems. Green plants, especially flowering plants, possess an incredibly large number of terpenoid compounds in comparison with other living organisms [24]. More than 80 thousand terpenoid secondary metabolites have been isolated from different sources in nature [25]. However, interestingly, there are a few hundred terpenoids that are present in almost all plant species, which are designated as primary metabolites and are involved in several biological activities, such as antioxidant, electron transfer, hormone and protein modification, membrane fluidity determination, etc., [24]. 'Terpenoids' are organic compounds commonly known as hydrocarbons. Their oxygenated, hydrogenated and dehydrogenated derivatives contain the general formula of (C\({}_{5}\)H\({}_{8}\))n, where 'n' suggests the number of isoprene units [26]. Natural products containing terpenoids are rich in \"isopentenyl pyrophosphate\", which is the active isoprene unit present in almost all living organisms as a common biosynthetic origin [27]. They are commonly classified as monotereponids (C\({}_{10}\)), sesquiterpenoids (C\({}_{15}\)), triterpenoids (C\({}_{20}\)), seasterterpenoids (C\({}_{25}\)), triterpenoids (C\({}_{30}\)), tetraterpenoids (C\({}_{40}\)), and polytrepenoids. Mono-, sesqui-, di-, tri-, and tetraterpenes are composed of two, three, four, six, and eight isoprene units, respectively. The unique structural diversity of terpenoids is employed in coupling chemistry, which essentially links C\({}_{5}\) isoprenoid precursors; C\({}_{5}\) dimethylallyl diphosphate and C\({}_{5}\) isopentenyl diphosphate become linked in a regular manner to generate C\({}_{10}\) geranyl diphosphate (GPP), which can be condensed with supplementary isopentenyl diphosphate entities to generate C\({}_{15}\) farnesyl diphosphate (FPP), C\({}_{20}\) geranylgeranyl pyrophosphate (GGPP), and C\({}_{25}\) geranylframesyl diphosphate, and this process continues [25]. For monoterepne synthesis, a distinct presence of geranyl pyrophosphate synthase enzyme is found in plants, although this enzyme is present in very low amounts in animals [28]. Terpene cyclases act directly on GPP, FPP and GGPP to yield mono-, sesqui-, and diterpenes; \"head-to-head\" linkage between FPP and GGPP yields tri- and tetraterpene presualene diphosphate and prephytoene diphosphate, respectively [29]. Within human physiological systems, the formation of these important terpene metabolites is significant for cellular protein modification through the process of glycosylation and energy generation [30; 31]. Several epidemiological studies have suggested that increased intakes of vegetables, fruits, whole-grains and pulses, and reduced intakes of red and processed meats and salt, are associated with minimized cancer risk [32]. As daily diets are responsible for the pathogenesis of multiple cancers, diets rich in plant-derived secondary metabolites can support the prevention of cancers. Terpenoids have been reported to possess several pharmacological activities, including against cancer (i.e., taxanes from _Taxus brevifolia_ and vincristine and vinblastine from _Catharantthus roseus_) [33; 34], malaria (i.e., artemisinin from _Artemisia annua_), and HIV (i.e., calanolide A from _Calophyll lanigerum_) [35], etc., In particular, in the case of breast cancer development, terpenids play chemopreviles by abrogating cancer initiation, promotion, and oxidative damage (Figure 1). Some terpenoid pigments including astaxanthin, bixin, and lycopene are widely used in various food industries. Additionally, the human body uses some of the terpenoids to yield essential compounds; for example, vitamin A is synthesized from \(\beta\)-carotene, a widely known plant terpene.