The description of the word you requested from the astronomical dictionary is given below.
The Julian Date (JD) counts the number of days since 12:00 UTC or 12:00 TAI on January 1st, −4712 on the Julian proleptic calendar. Don't confuse a Julian Date (which consists of a single number) with a date on the Julian calendar (which consists of three parts: a year, a month, and a day).
Julian Dates can have fractional parts. JD 2451545.25 indicates the instant of time that is 0.25 days (i.e., 6 hours) past 12:00 UTC or 12:00 TAI on January 1st, 2000 on the Gregorian calendar, i.e., 18:00 UTC or 18:00 TAI.