What are augments?

I told my minion Khem Val to synth me some pants, and three minutes later, I am notified that he has been "extremely successful", and has synth'd me a pair of "exceptional" pants.

The item now has an "augment" slot, and it looks to be modifiable in the same way that orange items are... but I don't see anything about augments when browsing the mods vendors - only hilts, barrels, enhancements, etc. No Augments.

So what is an augment? Where can I get them?

Augment schematics are gained from Slicing, and Armormech, Armstech, and Synthweaving can each craft some of them. They can't be bought from vendors, but you should find plenty of them on the GTN.

As to what they are? Well, as you've noticed, they're an item modification slot. They're automatically added when a companion 'crits' on their gear crafting task. You can also buy an Augment Kit (crafted by the above professions) and pay to augment your gear at an Item Modification Station.

Augments slots are automatically added to equippable items (armor shells, generators/focii, earpieces, implants, weapons, and relics) that were crafted with a critical success, which is rare.

Reverse engineering equippable items will give you Augmentation Slot Components that you need for crafting Augments Kits, which can be used at an Item Modification Station (AKA "Augment Table") to add an augment slot to any equippable item that doesn't already have one.

Augments themselves are separate items that go into the slot and give a single primary or secondary stat in addition to Endurance — either +18 stat +12 End or +18 End +12 stat. There are currently no Expertise augments in-game although they are in the assets.

Augmentation Kit schematics are learned from the Crew Skill trainer. Augment schematics come from Slicing, specifically Tech Parts missions. Both types of items can only be crafted by Synthweavers, Armormechs, and Armstechs; they all craft the kits and have the augments split between them.