How to choose an app name that's actually available
Apple won't let two apps share a name on the App Store, so the first check for any new app is whether someone already uses it. This app name checker searches the App Store for the exact name and for close variations, because "Nova" and "Nova: Daily Planner" count as the same name to most users even when Apple treats them as different listings.
The store is only half of it. People search for your app on Google, type your name into a browser, and look for you on social media. A name with a free .com or .app domain and a matching handle is far easier to market than one where every channel needs a different spelling. That's why the checker looks at the domains at the same time, and links straight to a full social handle check.
Keep the name short and distinctive. Both stores cap the name at 30 characters, and the words that describe what the app does are better spent in Apple's subtitle or Google Play's short description, where they still help search ranking. A two-syllable invented word, or a real word with a twist, is usually easier to own everywhere than a descriptive phrase.
Finally, check trademarks before you build a brand around the name. A free name in the stores doesn't stop a company with a registered trademark from asking Apple or Google to take your app down. Search the USPTO, EUIPO or UKIPO databases for your launch countries.