I would like to write something like 'A marker is mounted on the tail of the surgical instrument'. Is 'tail' a formal word to describe the other side of 'tip'? It sounds kind of informal to me and I'm looking for a formal word. Thanks for any help.
I am not a medic so you need to double-check but I believe the correct technical term is the "shank"
Note that your use of 'tip' is probably incorrect - it depends on the type of instrument.
Welcome to EL&U!
According to Merriam-Webster, a tip (in the sense described in the question) is
1 : the usually pointed end of something
[as in] a pencil tip
2 : a small piece or part serving as an end, cap, or point
Note that it states in neither definition which end of the object it is, simply that it is one.
Following this trail of thought we may make the following sentence:
Thread the thread through the tip of the needle with the eye, then use the opposing tip of the needle to penetrate the fabric, pulling the thread through with it.
In a similar fashion you could have, assuming the surgical instrument is a scalpel
A marker is mounted on the tip opposite to the blade of the surgical instrument.
Therefore you may continue to use tip.
Tail does not necessarily mean the rear end of e.g. an instrument, but may be
- the back, last, lower, or inferior part of something
(again from Merriam-Webster), which is the closest definition I could find.
As long as your instrument has a definite rear end you may also use tail, as in your proposed sentence.