Wikipedia
In Shin Buddhism, Shinjin (信心) was originally the Japanese word for the Buddhist concept of citta-prasāda (clear or clarified heart-mind), but now carries a more popular related meaning of faith or entrusting. According to Ueda, "shinjin is the mind of Amida Buddha given to and realized in a person. Shinran interprets shin (信) to mean truth, reality, sincerity; jin (心) means mind. When shinjin is realized, Amida's mind (wisdom and compassion) and the practitioners mind of blind passions become one."
Whilst a general Japanese Buddhist term it is particularly associated with the Jodo Shinshu teaching of Shinran. In that context it refers to the awakening and settlement of the mind that is aware of the working of Amida Buddha's Primal Vow, and the assurance of birth in the Pure Land at death:
In Notes on 'Essentials of Faith Alone' Shinran writes "Know that the true essence of the Pure Land teaching is that when we realize true and real shinjin, we are born in the true fulfilled land."