The documentation on the SAP Help Portal contains a full list of SAP terms and their definitions. However, a few of those terms may arise during implementation, so we'll review a handful below.
Client
In simple terms, a "Client" in SAP is the specific system a user logs in to. It is equivalent to an SAP Concur user logging in to a Production or Test environment. Many customers will have multiple SAP environments, such as Production, Training, Development, or QA. When users log in, each Client is represented by a 3-digit number.
Larger organizations may also have more than one operating unit. Each operating unit can have multiple Clients within it. Each Client can have an integrated SAP Concur entity.
You can review the image below to see an example. Widgets 'R Us is the company, it has two Business Units, each having more than one Client. Select the image to zoom in.

Master Data
Master data is the foundational data on which the database is built. It is generally considered static as it tends not to change regularly. Accurate master data is critical to the timely processing of transactions. Common examples that impact SAP Concur include employee master data and cost object data, though there may be others depending on the customer and their specific situation. The integration of this data is critical to the integration of SAP to SAP Concur systems.
When using Integration with Concur Solutions (ICS), a bulk load of master data is performed during the implementation process. Customers can define how much data is included in that bulk load. For example, if a customer has three company codes, but only one will be using SAP Concur, customers can choose to send only the master data for that one company code. The remaining data can be sent to SAP Concur at a later time if the customer chooses.
Once the bulk load is complete, the customer defines the schedule for how often the Master Data is updated in SAP Concur based on changes in the SAP system. SAP Concur recommends scheduling this job to run daily.
Transactional Data
With Integration with Concur Solutions (ICS), customers can control the amount and frequency of transactional data moved from SAP Concur to SAP. For example, if they have three groups all using SAP Concur, but only want the data from one to move to SAP through ICS, they can set this restriction. They can also control how often the data moves, from multiple times per day to on-demand.
The amount of data that is integrated is controlled in the SAP Concur configuration, and the frequency is controlled in the ICS configuration.