Every word in your app is part of a conversation with your users. Use this conversation to help them feel comfortable in your app.
Use familiar, understandable words and phrases. Technology can be intimidating. Avoid acronyms and technical jargon that people might not understand. Use what you know about your audience to determine whether certain words or phrases are appropriate. In general, apps that appeal to everyone should steer clear of highly technical language. Such language may be appropriate in apps that target a more advanced or technical crowd.
Keep interface text clear and concise. People absorb short, direct text quickly and easily and don’t appreciate being forced to read long passages to accomplish a task. Identify the most important information, express it succinctly, and present it prominently so people don’t have to read too much to find what they’re looking for or figure out what to do next.
Identify interactive elements appropriately. People should be able to tell at a glance what an element does. When labeling buttons and other interactive elements, use action verbs, such as Connect, Send, and Add.
Avoid language that might sound patronizing. Avoid we, our, me, and my (for example "our tutorial" and "my workouts"). They're sometimes interpreted as insulting or patronizing.
Strive for an informal, friendly tone. An informal, approachable style echoes the way you speak with people over lunch. Use contractions occasionally, and you and your to address the user directly.
Be careful when using humor. Remember that people are likely to read the text in your interface many times, and what might seem clever at first can become irritating over time. Also remember that humor in one culture doesn't necessarily translate well to other cultures.
Use relevant and consistent language and imagery. Always make sure guidance is appropriate for the current context. If someone’s using an iPad, for example, don’t show iPhone tips or graphics. Use language that’s consistent with the platform. You tap, flick, swipe, pinch, and drag content on the touchscreen. You press physical buttons and content that responds to 3D Touch. You rotate and shake the device.
Refer to dates accurately. It’s appropriate to use friendly terms like today and tomorrow, but these terms can be confusing or inaccurate if you don’t account for the current locale. Consider an event that starts just before midnight. In one time zone, the event may start today. In another time zone, the same event may have started yesterday. Generally, dates should reflect the time zone of the person viewing the event. However, in some cases, such as in a flight tracking app, it may be clearer to explicitly show the start date and time zone where the flight originates.