An occurrence is in its widest sense anything that happens. This includes human activities, but also physical and (bio)chemical processes, control processes, events and physical phenomena. Processes includes both discrete processes and continuous processes. A discrete process consists of a sequence of discrete process steps or actions. Examples of discrete processes are business processes in which various persons prive a contribution in a process, mechanical production processes in the mechanical industry and batch processes in the chemical industry. A continuous process is a process that is an operation on a continuous stream of raw material to produce one or more continuous streams of product materials. Examples of continuous processes are the flowing of water in a river, the operation of an electric power generation plant and the various refinery processes. An occurrence in fact always happens over time, but especially for events the duration often can be neglected. All these kinds of occurrences can be modelled in Gellish.

Occurrences, activities, processes and events can be classified by nouns as well as by verbs, such as the classification of a human activity as a walk or as walking. Both kinds of terms are included in the Gellish Dictionary. Most kinds of occurrences have an active form, a passive form and a noun form and sometimes other forms in the dictionary. For example: act, acting and action, but also activity. All such forms are treated as synonyms in Gellish English. Each individual activity, process or event shall be classified explicitly, for ezxample as follows:

An occurrence, activity, process or event describes the behaviour of the objects involved in that occurrence as well as their mutual interaction. This means that an occurrence is an interaction between two or more objects. Therefore, an occurrence can be expressed in Gellish English as a higher order relation, being a relation with usually more than two related objects. The details of an occurrence are expressed by the specification of the objects that are involved in the interaction, especially in the various roles that they play.

An occurrence is a higher order relation, because an occurrence is an interaction of several objects. This means that several objects are involved in an occurrence, each with its own role, although often only one or two objects with their roles are modeled. The roles that objects play in an occurrence is modeled in Gellish by the specification of a number of facts, each of which specifies that an object plays a particular role in the occurrence, whereas each fact is specified on a separate line in the database table. These involvement relations are described by expressions that use relation types that are subtypes of the <is involved in> relation type.
For example:

The Gellish English Dictionary contains a large number of relation types, to describe ways in which objects can be involved in occurrences, processes and activities. For example:

• is performer of
• is enabler of
• is input in
• is output of
• is subject in
• is tool in
• is enabler of
• is controlled by
• etc.

If only the two roles are of interest, the roles of the performer and of the subject, then it is allowed in Gellish to express the occurrence as a binary relation. For example, the fact that pump P-101 pumps fluid S-1 can be expressed as follows:

The UID of the fact (in the example fact 101) is the identifier of the occurrence. In this case the occurrence is not given a name, but as a guideline it is recommended to use the concatenation of the relation type and the subject as its name, so that the name of the process in the example would be ‘pumping of S-1’.

Occurrences that do not exist yet in the Gellish English dictionary can be added in a similar way as defining physical objectsby obeying the [wiki:“Proper definition of a concept” rules for proper definitions] in Gellish English. A kind of occurrence shall be defined as a subtype of an existing kind of occurrence and facts that are by definition true shall be added to complete the definition.
In general, generic kinds of activities and processes are independent of the subject of the occurrence, and independent of the performer or the enabler. For example, the activity 'design' or 'operate' does not include an indication of what is designed or operated. However, more specialised ‘qualified’ activities often do include the subject of the activity or process. For example, “design a pump” or “operate a facility”. In such a case the definition of such a kind of occurrence should include and explicit relation that states that it has by definition as subject a particular kind of thing. For example:

Similar considerations apply for specific activities that include a specific subject. For example, the specific activity called 'design P-101“ is by definition a design of a thing called P-101. This is specified as follows:

Often it is required to specify in which sequence activities or chains of shall be or are performed. This may result in activity networks. For example, in activity scheduling for testing, commissioning and start up of facilities or to describe a maintenance procedure. This can be done for sequences of kinds of activities, but also for sequences individual activities, or for procuderes in which typical activity sequence is describedthat shall be repeated every time that the procedure is followed.
For example, the following activity sequence may describe steps that shall be taken during a planned maintenance shut-down (also called a turnaround) of facility:

Whereas a sequence of individual activities is expressed as follows:

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