retain (something)

"Retain" is a formal word which means "keep". Here are some examples of sentences in which you can use "retain":

Many nursing mothers retain much of the weight that they gained during pregnancy.

Tests like these just measure your ability to acquire and retain knowledge. They're not a true test of a person's skill or expertise.

Here are some of the most common things that are "retained':

(someone) retains power

(a company) retains employees

(something) retains heat

(something) retains moisture

(someone) retains ownership of (something)

a receipt

A "receipt" is a piece of paper (or maybe an electronic document) which shows how much someone has paid for something. You get one of these when you buy something.

A common practice at stores in the United States is to allow customers to return items to the store that they don't want, if they bring the receipt to prove that they bought the items at that store.

for your records

Your "records" are a collection of important information. 

Here are some things that people sometimes keep in their personal records:

• medical records
• bank statements
• birth certificates
• school transcripts
• receipts for business expenses

Some important documents like tax forms come with several copies. One of the copies may be labeled "For Your Records".