Common time

The pedant is a disturbingly-slender,
reedy-voiced, scrutinous man.

There's a lot of confusion about rhythm, making the subject an easy target for low-grade, often barely-informed pedantry. See any internet music forum, or rehearsal of college age players, or gathering of amateur musicians.

Your hard-core rhythm pedant doesn't care if it's easy for people to do actual musical work, he cares about textbook precision— his version of it, anyway, because he may actually know jack squat on the subject. He just likes bringing conversations to a screeching halt and changing the subject to one he's comfortable with: music theory pedantry.

Sure to drive such people bananas is the useful idea of common time functional equivalence— referring to rhythm functions by their values in 4/4.

Most drummers learn early in their education that:

Quarter notes = “one beat”
Half notes = “two beats”
Whole notes = “four beats”
8th notes = the hihat rhythm— a two-note subdivision
16th notes = hand-to-hand fast notes— a four-note subdivision
Triplets = the triplety things— a three-note subdivision

And so on. Simple, obvious references for rhythmic values. Students will be screwed for really understanding rhythm if they get too attached to them, but they're usable for initially getting it.

As you learn more, you discover that all of that always depends on the time signature. There is no blanket term for a one-beat rhythm, or for a two, four, or three note subdivision. For example:

In 2/2 (or cut time), a half note = one beat
In 12/8, a dotted quarter note = one beat
In 3/8 counted “in 3”, an 8th note = one beat  

In 2/2, a quarter note = a two-note subdivision
In 3/4 counted “in 1”, a quarter note = a three-note subdivision
In 12/8, a quarter note = a three-note subdivision over two beats, the equivalent of a quarter note triplet in 4/4.  

In 4/4, a three note subdivision = 8th note triplets
In 6/8, a three note subdivision = 8th notes

So, to talk about ordinary rhythmic concepts like beats and subdivisions, we either have to refer to the correct rhythm for the current meter (and interpretation of that meter), or we have to say two/four/whatever-note subdivision of the beat, and get into rhythmic terms that most people don't know. It's annoying, and tends to confuse students, and readers.

So for the sake of having a functional language for rhythm, I often use the common understanding of those rhythms, and refer to them by their values in 4/4.

This is not just for convenience because everyone is so poorly educated about rhythm; it's also an objective reference. 4/4 is called common time not just because a lot of music is written in it, but also because it's the native meter* for our system of notation. The whole note is the fundamental rhythmic value from which all the others are derived, and a whole note = one whole measure of 4/4 time. For our system of rhythm and meter, 4/4 is normal.

We're using 4/4 as a reference point in a similar way to other instruments using the major scale as a reference point when talking about modes and other types of scales. 

The point of this is that in drumming, we have some common rhythmic functions like the beat, and the two note subdivision, and the four note subdivision, and the three note subdivision, and quarter note, 8th note, 16th note, and triplet, respectively, are the best known words for those things.

So regardless of meter, I may refer to notes of a four note subdivision as “functioning like 16th notes.” That comes up most frequently in 2/2, where 8th notes are the four note subdivision. As I mentioned in this post, 12/8 inherently uses a three note subdivision, which I'll refer to as a “triplet feel”, and refer to the parts of individual beats of 8th notes the same way I do 8th note triplets in 4/4.

You do have to educate people on the correct terms and theory, so I don't make these references casually. That's what hacks do. I use the words feel and functioning as, and explain what is meant by them.

* - Meter = time signature = time. The terms are practically interchangeable. I prefer to say meter. Time signature sounds like it refers to the written indication that tells you what meter the piece is written in, but I don't know any musicians who make that distinction. Musicians say time signature or time or meter or meter signature.