Samatha: 1. Concentration meditation: Attending to a single object of meditation without interruption. 2. The Noble Eightfold Path: Way to the End of Suffering, Chapter Seven, ‘Sammā-Samādhi’, Bhikkhu Bodhi.
Three Characteristics: (ti-lakkhana) 1. The Three Characteristics of existence, or signata, are Impermanence (anicca), suffering/dissatisfaction or misery (dukkha) and Not-Self (anattā). Buddhist Dictionary by Nyanatiloka. 2. In Buddhism, the three marks of existence are three characteristics of all existence and beings, namely impermanence (anicca), unsatisfactoriness or suffering (dukkha), and non-self (anattā). These three characteristics are mentioned in verses 277, 278 and 279 of the Dhammapada. That humans are subject to delusion about the three marks, that this delusion results in suffering/dissatisfaction, and that removal of that delusion results in the end of suffering/dissatisfaction, are central themes in the Buddhist Four Noble Truths and Noble Eightfold Path. Wikipedia.
Abbreviations in footnotes: AN: Aṅguttara Nikāya, DN: Dingha Nikāya, MN: Majjhima Nikāya, SN: Saṃyutta Nikāya