Helpful definitions

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Term Definition
Active user By default, an active user is a user who has logged at least one event during your selected interval: real-time (five minutes), hourly, daily, weekly, or monthly. You can edit this definition by marking certain events as inactive. Users who have only performed inactive events would no longer be counted as active users.
Amplitude ID A unique ID used to identify a particular user across multiple User and/or Device IDs. 
Behavioral cohort A set of users with shared behaviors and properties, e.g. users who have made at least five purchases within the last 30 days.
Device ID A device-specific identifier. For iOS, the Device ID is the identifierForVendor (IDFV). This does not persist across installs. For Android, we generate a random Device ID that gets stored locally on the phone. However, there is a flag to use the Advertising Identifier as the Device ID if you want. The Advertising Identifier does persist across installs but users have the option of changing this identifier on their devices at any time. See a detailed explanation for how the value is obtained here.
Event An action a user takes in your product. This could be anything from pushing a button, completing a level, or making a payment. Depending on your product, you should aim to track between 15 and 200 events to get a full understanding of how users are engaging with your app.
Event ID A counter distinguishing events. Event IDs are associated with the device ID and are not unique to an organization. For example, device ID XYZ could have event IDs of 1, 2, 3, 4, etc. as the device fires those events. Another device ID, ABC, will also have event IDs of 1, 2, 3, 4, etc. as that particular device fires events.
Event property Reflects more detailed information about an event at the time it was tracked. These properties depend on the type of app you have and the specific information you think is necessary for understanding a particular event. General keys we have seen are cause, description, category, type, duration, level, % completed, name, count, source, status, from, number, lives, authenticated, error, rank, action, and mode.
Funnel A customized series of events a user goes through to successfully do something in your app, such as successful onboarding.
Inactive event

Events are configured as inactive in Amplitude Data.

Amplitude users typically configure inactive events as events that aren't tied to an action done by the user but still occur within the app or website. One such example is a push notification or email sent to the end user.

Monthly tracked user (MTU) A unique user who triggers one or more events in a project within a calendar month.
New user A new user is one who has logged an event for the first time (this includes inactive events). The time of when the user is new is the time of when they logged their earliest event.
Retention Measures how often your users are returning to your app over some timeframe.
Session A period of time when a user has your app in the foreground. By default, events within five minutes of each other are combined into a single session on a mobile app. On web, events within 30 minutes of each other are combined into a single session.
Session ID Represents the number of milliseconds since epoch (UTC) of when the first event of the session occurs.
Stickiness Measures the number of different days/weeks a user performs an event in each week/month interval.
Unique user A distinct individual to whom events are attributed. Amplitude uses a system of identifiers to count unique users. In the raw data, unique users are counted using amplitude_id
User ID A unique ID used to identify a user, e.g. email address or username.
User property Reflects traits about the individual person using your app. Examples of custom user properties are locale, referral source, plan type, number of photos uploaded, number of units of in-game currency, and current level in a game. The most important thing to remember is that these user properties reflect the state of the user and apply across all their events.
amplitude_logo.png A prefix Amplitude uses to mark user properties it automatically tracks through its SDKs.
amplitude_logo.pngAny Active Event Any event you are tracking in Amplitude that has not been marked as inactive.
amplitude_logo.pngAny Event

Any event you are tracking in Amplitude, regardless of activity.

amplitude_logo.pngTop Events for Segment The top 10 active events by event totals at the time of the query for the specific segment (in the chart's right module). 
amplitude_logo.pngTop Global Events

The top 10 active events by event totals at the time of the query.