It’s quite often I find myself educating people on the difference between an Interstitial, a Popup and a Modal.  So much that I thought a quick and easy definition of each could help others.  The difference of each are significant, and the strategies around each are important for organizations to understand to reduce confusion and creep in projects.

Interstitial

Simple Definition: An interstitial is a temporary experience (page or modal) from a UI perspective before the customer is redirected to a desired landing page. Interstitial typically have a moving object (called a Throbber) that shows the customer that something is about to happen, or your system is working.  Probably one of the most popular Interstitial is Priceline.com that occurs after your give your flight search requests.

Strategy:  Typically you show this when you have a longer response time to a long API call or system action (so they don’t think the website is stalled), but it you can also use it in a time where customer education or advertising (no CTA) is needed.

Popup

Simple Definition:  A new window based on action that prioritizes over active tab or window, typically sized smaller than your usual new tab experience.

Strategy:  Yeah, these are dying (appropriately) due to browsers getting smarter with their Popup blockers, but they can still be effective tools for customer education, or if you’d like to keep your brand on your customers mind when forwarding your customers onto another website as a referral.

Modal

Simple Definition:  A modal is a scripted effect that allows you to overlay an element over focused content.  Typically a modal frosts the existing window to show that another message needs to be read.

Strategy:  Primary tactic is to make sure that the customer sees an important message because of an action on page, but can also be used to show progression in a conversion flow.  For ex: Need to ask a customer for their zip to calculate shipping, but can’t put it inline, or don’t want to move from the cart?  …throw a modal. Proactive modals based on customer data or actions are also a smart way to encourage email signups, personalization or cross sells.
Hope this helps.  If it doesn’t, then congrats ..you are too cool for school.