'To touch on' definitions:

Definition of 'To touch on'

From: GCIDE

Touch \Touch\, v. i.
1. To be in contact; to be in a state of junction, so that no space is between; as, two spheres touch only at points. --Johnson. [1913 Webster]
2. To fasten; to take effect; to make impression. [R.] [1913 Webster]
Strong waters pierce metals, and will touch upon gold, that will not touch upon silver. --Bacon. [1913 Webster]
3. To treat anything in discourse, especially in a slight or casual manner; -- often with on or upon. [1913 Webster]
If the antiquaries have touched upon it, they immediately quitted it. --Addison. [1913 Webster]
4. (Naut) To be brought, as a sail, so close to the wind that its weather leech shakes. [1913 Webster]
To touch and go (Naut.), to touch bottom lightly and without damage, as a vessel in motion.
To touch at, to come or go to, without tarrying; as, the ship touched at Lisbon.
To touch on or To touch upon, (a) to come or go to for a short time. [R.] [1913 Webster]
I made a little voyage round the lake, and touched on the several towns that lie on its coasts. --Addison. [1913 Webster] (b) to discuss briefly, as only a small part of a discourse. [PJC]