What is the difference between retinol and retinaldehyde?

Retinaldehyde (retinal) is one step closer to active retinoic acid in the skin's conversion pathway than retinol, which reviewers and dermatologist-led YouTube channels describe as making it faster-acting at comparable concentrations while often being better tolerated. Retinol is more widely available and better studied in consumer formulas, while retinal products like the Meditherapy serum are a newer category.