The practice of optimizing mentions of your website, product, or service across the web. Off-page or off-site SEO refers to optimizing content on sites other than your own.
Off-page optimization can refer to web citations or backlinks. All backlinks are technically citations, but not all citations are backlinks. Citations can simply be a mention of your business name, address, product, service, or other contact information listed on a 3rd party website. Backlinks refer to links pointing to your website from 3rd party websites.