The definition of an Affirmation sentence, quote or statement 

Affirmation is the practice of positive thinking and self-empowerment—fostering a belief that "a positive mental attitude supported by affirmations will achieve success in anything. In other words  the process of manifastation. More specifically, an affirmation is a carefully formatted statement that should be repeated to one's self and written down frequently. For affirmations to be effective, it is said that they need to be present tense, positive, personal and specific.

Scientific researches

A 2009 study found that present tense positive affirmation had a positive effect on people with high self-esteem, but a detrimental effect on those with low self esteem. Individuals with low self-esteem who made present tense (ex. "I am") positive affirmations felt worse than individuals who made positive statements but were allowed to consider ways in which the statements were false. Individuals with low self-esteem who made future tense affirmations (ex. "I will") saw positive effects.Some studies have found that self-affirmations, which involve writing about one's core values rather than repeating a positive self-statement, can improve performance under stress. An fMRI study in 2016 demonstrated the role of two reward and valuation brain regions (ventral striatum and ventromedial prefrontal cortex) as primary pathways associated with self-affirmation. Self-related processing and prospection associated regions in the brain were further associated with objectively measured positive behavioral changes following future-oriented self-affirmations.