The following terminology is used for approval processes in Salesforce.

Approval Actions

An approval action occurs when a step is approved by all the required approvers.

Approval Process

An approval process automates how records are approved in Salesforce. An approval process specifies each step of approval, including who to request approval from and what to do at each point of the process.

Approval Request

An approval request is an email, Salesforce1 notification, Lightning Experience notification, or Chatter post notifying the recipient that a record was submitted for approval and that his or her approval is requested.

Approval Steps

Approval steps define the chain of approval for a particular approval process. Each step determines which records can advance to that step, who to assign approval requests to, and whether to let each approver’s delegate respond to the requests. The first step specifies what to do if a record doesn’t advance to that step. Later steps specify what happens if an approver rejects the request.

Assigned Approver

The assigned approver is the user responsible for responding to an approval request.

Delegated Approver

A delegated approver is a user appointed by an assigned approver as an alternate for approval requests.

Email Approval Response

Email approval response lets users respond to approval requests by replying to an email notification.

Initial Submission Actions

An initial submission action occurs when a user first submits a record for approval. By default, the record is locked.

Final Approval Actions

Final approval actions occur when all required approvals have been given for a record.

Final Rejection Actions

A final rejection action occurs when an approver rejects the request and it goes to the final rejection state.

Outbound Message

An outbound message sends information to a designated endpoint, like an external service. Outbound messages are configured from Setup. You must configure the external endpoint and create a listener for the messages using the SOAP API.

Process Instance

A process instance represents one instance of an approval process. A new process instance is created each time a record is submitted for approval.

Process Instance Node

A process instance node represents an instance of an approval step. A new process instance node is created each time a record enters a step in an approval process. No process instance node is created when the record doesn’t meet the step criteria or if the approval process instance is otherwise completed without entering the step.

Recall Actions

A recall action occurs when a submitted approval request is recalled. By default, the record is unlocked.

Record Locking

Record locking prevents users from editing a record, regardless of field-level security or sharing settings. By default, Salesforce locks records that are pending approval. Only admins can edit locked records.

Please check with below links for reference.
 https://www.adminhero.com/creating-your-first-approval-process/
 https://developer.salesforce.com/page/Approval_Processes