It's a good idea before having any discussion to define the terms. We can then be sure the words and symbols we are using mean the same thing to everyone involved in the discussion. I ask all of my guitar students to know these terms so that we can communicate clearly in our guitar lessons.
To avoid confusion, we name the fingers of the left and right hand differently on the guitar.
The right hand fingers are named after the Spanish names for the fingers which are in parentheses below. They are abbreviated with a single letter.
thumb = p (pulgar)
index = i (indice)
middle* = m (mayor or medio)
ring = a (anular)
pinky* = c or s (chiquito)
*different dialects of Spanish may call the fingers by different names. Sometimes the middle finger is called corazón; sometimes the pinky is called meñique. Don’t be confused by this, because the way we name the fingers on the guitar is standard.
The left hand fingers are numbered 1 to 4, starting with the index. The thumb is generally not used, however, I list the thumb as T because it is sometimes used in modern tablature for pop songs that way.
thumb = T
index = 1
middle = 2
ring = 3
pinky = 4
no finger or open string = 0
When making annotations in musical scores, it is common to write the fingers to be used into the score. Here is an example of this, from my working copy of Bach’s Cantata BWV 140, 4th movement.