This is a non-exhaustive list of some Bubble terms you may see in Core Reference. Generally this will only feature terms that have Bubble-specific meanings and will not define technical terms used in their normal sense.
A step that a Bubble workflow takes, i.e. the different kinds of things that workflows can do
Examples include sending an email, logging the user in, showing an element, hiding an element, etc.
Bubble elements can store bits of data on themselves - this data only exists when the page is loaded (i.e. not in the database), and resets when the page refreshes
For example, you could have a group that shows different information depending on the tab that's currently active - the group could have a custom state to keep track of which tab is active, which would then influence the text that shows up
You can create custom states on an element and set their initial values via the "i" icon at the upper right of the property editor when you have the element selected
You can edit the custom state of an element via the workflow action "Set state"
A feature that lets outside apps access the data in your Bubble app via API calls
Allows querying your data as well as creating / updating / deleting data
A collection of Things in the database that share the same fields, which often semantically refers to a cohesive idea an app wants to represent
Slightly more technical: a collection of records with the same fields
For those familiar with spreadsheets: a data type is kind of like a single sheet in a spreadsheet file
More technical: an "object", or for those familiar with relational databases, a table
For example, for a project management app, there might be the "Project" and "Task" data types
Built-in Data Type: there is only 1 built-in data type which comes with all Bubble apps: User
Unlike Custom Data Types, Users 1) cannot be created by the "New Thing" action; 2) have some extra built-in fields like "email" and "is logged in"; and 3) can be extended with additional extra fields via connecting with a social network or oauth provider
Custom Data Type: all the other data types of an app, which an app creator creates and defines
The app creator determines which fields each Custom Data Type has (i.e. which columns exist on that table)
What type of information is stored in a field of a Data Type
There is a built-in set of Field Types that all Bubble apps have, including numbers, text, geographic addresses, etc.
A field can have a data type as its type - for example, in a classroom management app, for a given class, the Teacher can have "User" as the field type
Fields can also be "a list of" a given type - for example, in the classroom management app, for a given class, the Students can have "list of Users" as the field type
To learn more, see this guide on creating a data structure
Edit-mode - the state of your app when you are editing it on bubble.io
Run-mode - the state of your app when your end-users are visiting your app (you can get from Edit Mode to Run Mode by clicking on the "Preview" button in the editor)
Operator (formerly known as "Message")
A transformation / manipulation that can be applied to data in a dynamic expression
These can be chained together to perform a computation
For example, in the dynamic expression "Current User's email:number of characters > 5", the following are all operators: "email", "number of characters", ">5"
When building a dynamic expression, the operators available at any point depends on the dynamic expression as written thus far; for example, the ":number of characters" operator makes sense when attached to some text, but not when attached to a number
When you app is actually running and you have users engaging with it, i.e. when you're "running" your app, not in the editor
When you click "Preview" in the editor, that simply opens up the app in run mode, just as if you had visited the URL of your app
Both the "Development" version and the "Live" version of your app have run modes, i.e. if you are editing your app on the Development version and click "Preview", you will enter run mode of the Development version
Contrasts with "Edit-mode" (see above)
A set of properties about the look-and-feel of an element that can be reused on other elements of the same type (e.g. Styles A and B can apply to any buttons in your app, Style C can apply to any text elements in your app, etc.)
These are edited via the Styles tab, and can be applied to a particular element via the Property Editor
New Bubble apps come with a default set of Styles., i.e. a default Theme (see Theme)
Styles help with consistency in the look-and-feel of an app, and if you have a large app, using Styles instead of applying custom individualized look-and-feel properties to each item could result in a performance improvement