First aid was medical care offered by someone other than a medical professional, sometimes via a first aid kit. (DS9: "The Visitor")
In 2371, after The Doctor gave Kes information of first aid for burns to study, she expressed her interest in learning about anatomy and physiology rather than just first aid. (VOY: "Eye of the Needle")
In 2374, an injured Jadzia Dax assured Worf that she could administer her own care until he returned, saying she had taken basic first aid. (DS9: "Change of Heart")