Some Definitions

Original Instruments - or, as often seen on recordings, "On original instruments." This could possibly be true, if strings alone are used. For groups with winds, the phrase "on period instruments" would be more precise, as period instrument groups almost universally use copies of historical winds. There are difficulties in restoring historical winds to playing condition and keeping them in working order without destroying their value as antiques.

Period Instruments - original instruments and copies thereof.

Authentic Performance - "Authenticity" in the performance of old music is a term with intuitive appeal, but because of intellectual attacks it is out of favor ... more on this later.

Historically Informed Performance (HIP) - self-explanatory (I think ... if you disagree please comment). The somewhat unfortunate acronym is not intentionally smug (usually).

Early Music (EM) - often a synonym for use of period instruments or historical techniques. More date-focused usage always includes pre-Baroque music, but may omit the Baroque period. The Classical period (roughly coincident with Mozart and Beethoven) is more often omitted.

The preceding terms can often be used quite interchangeably.

Baroque - in music, roughly 1600 to 1750. One Internet wag was more precise, opining that the Baroque dawned on Jan 1, 1600 local time.

Classical - familiarly used to denote Western art music generally. The Classical era, though, is generally regarded as the period from 1760 to 1820, roughly the span from Haydn through Beethoven.

Baroque Orchestra - code word for a group using period instruments, larger than a chamber group, and with instrumentation generally suitable for many works from Biber (and earlier) to Bach. Why do so many Baroque orchestras eschew 17th C. music in favor of Mozart and Beethoven? That's a topic for a later post.