A pair of antique reading glasses given to James Kirk by Leonard McCoy as a birthday present, because Kirk is allergic to the standard treatment for failing eyesight.
In Star Trek II: The Wrath of Khan, McCoy visits Kirk on his birthday and gives him a pair of centuries-old spectacles. Most people with ageing eyes are prescribed Retinax V, but Kirk cannot take it. He uses the glasses during the film, notably to read out a prefix code, and they are cracked in the battle with Khan.
The glasses return in Star Trek IV: The Voyage Home. Stranded without money in San Francisco in 1986, Kirk sells them to an antique dealer for one hundred dollars. When Spock points out that they were a gift from McCoy, Kirk answers that they will be again.
The remark implies a neat loop in time, in which the spectacles McCoy later buys are the very pair Kirk sold.