There are several synonyms for the word "certainness" that can be used in different contexts. The most common synonyms include confidence, conviction, assurance, surety, and certainty. Confidence refers to a feeling of trust and belief in oneself or another person's abilities. Conviction implies a fixed or firm belief that something is true or right. Assurance refers to a promise or guarantee of something being done or provided. Surety means a promise or pledge to make good on something. Finally, certainty indicates a high level of knowledge or proof that something is true or likely to happen. These words can be interchanged depending on the situation, but all convey the idea of having no doubt or uncertainty.