Tsadi is the eighteenth letter of the Hebrew alphabet.

The gematria of Tsadi is 90.

In Ethics of Our Fathers it says: “When one reaches the age of ninety, one is bent over (lashuach).” On a physical level, this means that at ninety, a man is infirm and bowed with weak­ness. On a spiritual plane, it represents the concept of humility. When one reaches ninety, he has become so spiritual and humble that he bends over for G‑d. He is no longer an inde­pendent character but an extension of G‑d Himself.

At the age of ninety, one has achieved a heightened level of prayer. He has the ability to feel a direct connection to G‑d when he prays. Additionally, it is explained in the Midrash Shmuel that the word lashuach means “to pray constantly.” That connection is the foundation of a tzaddik. A tzaddik exists not for his own benefit, but to serve as an offshoot of G‑d. We go to tzaddikim to pray on our behalf because we know that the prayers of the tzaddik will be answered.