What does "article" mean in this?

cauterize: To burn and hence seal open tissue using a heated article or caustic agent so as to stop bleeding or minimize the risk of infection.

If you are familiar with an "article of clothing", it's the same meaning here.

"An article" is a single instance of a thing of a particular category.

Ex:
"We ask that our guests leave any articles of value inside the provided lockers"
"There was a single article of clothing left on the ground."

Here, the category would be something like 'medical instruments'.

Usage notes:

Apart from "article of clothing", which is almost like a set phrase, "article" is very formal and somewhat uncommon. Furthermore, many nouns cannot be used naturally with "article".

For example, an "article of pizza" is grammatically correct, but sounds very odd.