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What did Thomas Aquinas mean by esse, or the act of existing?

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St. Thomas Aquinas Enthroned Between the Doctors of the Old and New Testaments, with Personifications of the Virtues, Sciences, and Liberal Arts, fresco by Andrea da Firenze, c. 1365; in the Spanish Chapel of the Church of Santa Maria Novella, Florence. — Thomism *St. Thomas Aquinas Enthroned Between the Doctors of the Old and New Testaments, with Personifications of the Virtues, Sciences, and Liberal Arts*, fresco by Andrea da Firenze, c. 1365; in the Spanish Chapel of the Church of Santa Maria Novella, Florence.

For Thomas Aquinas, esse refers to the "act of existing". He utilized this concept to establish a fundamental distinction between God and all other beings.1

In Thomistic thought, creatures are composed of both essence and existence. A creature’s essence—what it is—does not inherently include the act of existing; therefore, creatures participate in esse only to the extent that their specific nature permits. Because they cannot account for their own existence, finite beings are dependent and point toward a sustaining cause.12

In contrast, God is defined as "pure being," meaning that in God, essence and existence are identical. God is the unconditioned source of existence, possessing esse as his very nature rather than receiving it through participation. By conceiving of being as an active, dynamic "act" rather than a static state, Aquinas provided a framework to explain how finite things are constantly sustained by the infinite.12