Terpenes (/ˈtɜːrpiːn/) are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, and by some insects. They often have a strong odor and may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores.  (Wikipedia)

You already know what terpenes are because you’ve experienced them all your life. Simply put, terpenes are what gives an orange its citrusy smell. They give pine trees their unique aroma. They’re even responsible for the relaxing effects in lavender. They are chemicals that determine how things smell.  (https://www.analyticalcannabis.com/articles/the-difference-between-cannabinoids-and-terpenes-311502)