Finally, we know what metadata is, or perhaps more precisely, how the Government is choosing to define it as part of its controversial Data Retention Bill.

If the legislation passes the Senate, it will be the first time metadata has been defined by Australian law.

So what is it?

Metadata is the information that identifies us via the phone calls we make and our active online lives.

When put together it can reveal a detailed picture of an individual's identity and their relationships.

Essentially, it is an electronic x-ray of the when, where, how and to whom we communicate with over our phones and via the internet.

According to Privacy International the amount of data that exists in the digital realm today is about ten times that which existed less than a decade ago.

As new technologies come online, the scope of what metadata is grows and that is why defining metadata at any particular point in time is fraught.

Nigel Phair, a former AFP police officer and cybercrime specialist, said even he struggled to define what metadata was.

"I've tried to define metadata and I couldn't, so I started to approach the definition by what metadata isn't."

Mr Phair said defining metadata in legislation would likely create problems down the track as new technologies came online.

"The cybercrime act in 2001 lists floppy disks and we haven't used those for aeons, so the more prescriptive we are with metadata definitions the harder it is to apply the legislation in practice," he said.

"We don't know what's down the track in terms of new digital devices and even artificial intelligence."

Jon Lawrence from Electronic Frontiers Australia disagrees and says it is necessary to define metadata in the legislation.

"In this particular circumstance it's about the indiscriminate invasion of privacy of all Australians and the scope of the metadata should not be allowed to creep without proper parliamentary oversight," he said.

Inspector Gavan Segrave from Intelligence and Covert Support Command acknowledged to the Parliamentary Joint Committee on Intelligence and Security that Victoria Police would like to have access to web browsing histories.

It is this type of "scope creep" that concerns Mr Lawrence.

"Without a definition, the scope of metadata would likely increase to include web browsing histories, and I'd consider that to be active content surveillance," he said.

What's not included?

Web browsing history.

Web browsing history. The body or text of SMS messages.

The body or text of SMS messages. The body and subject lines of emails.

The body and subject lines of emails. Files attached to emails including photos or documents.

Files attached to emails including photos or documents. The audio of phone conversations.

The audio of phone conversations. The audio recordings of online or social media chats.

The audio recordings of online or social media chats. Continuous location tracking via mobile devices.

What will be kept for a minimum of two years?

Phone calls

Incoming caller identification.

Incoming caller identification. Outgoing caller identification.

Outgoing caller identification. The time, date and duration of the phone call.

The time, date and duration of the phone call. The location of the device at the beginning and the end of the phone call.

The location of the device at the beginning and the end of the phone call. The unique identifier number assigned to a particular mobile phone device.

The unique identifier number assigned to a particular mobile phone device. The status of a mobile device, for example, if it is lost, stolen or on roaming.

The status of a mobile device, for example, if it is lost, stolen or on roaming. Information about what features were used on any particular call such as call waiting or call forwarding.

Internet (if these services are provided by an Australian operator)

Email address.

Email address. The time, date, size and recipients of emails.

The time, date, size and recipients of emails. The file type and size of any attachments sent or received with emails.

The file type and size of any attachments sent or received with emails. Online chat time, date and the identity of those on the chat.

Online chat time, date and the identity of those on the chat. Details about internet usage including how much bandwidth the internet service provides.

Details about internet usage including how much bandwidth the internet service provides. How many uploads/downloads made and the size of each one.

How many uploads/downloads made and the size of each one. Details about what technology enabled each communication i.e. ADSL, wifi, cable internet.

Details about what technology enabled each communication i.e. ADSL, wifi, cable internet. Account details held by the ISP or telco provider; including when the account was activated or suspended.

Privacy advocates concerned over agencies sharing metadata

Alone, metadata is not particularly useful in solving crime, but when combined with the metadata of the entire population and ran though sophisticated pattern-matching computers - the kind used by intelligence agencies and the AFP - the value of metadata and its capacity increases dramatically.

"Metadata is much more valuable to an investigation than content. We learn much more through an individual's metadata than we would from their conversations," Mr Phair said.

And that is what concerns privacy advocates.

Internet Society of Australia chief executive Laurie Patton said up until now law enforcement agencies have had to find a way to identify their source, but this legislation would make things much easier for law enforcement while posing great risks to all Australians.

"At the moment metadata is stored in literally hundreds of places because there are hundreds of ISPs and telcos. This bill will require the ISPs and telcos to pull together our metadata creating what is known as a honey pot," he said.

"So metadata which was spread across a whole range of servers in various locations will be pulled together and it becomes far more powerful when it can be amassed and subjected to highly sophisticated computer algorithms that have been developed and shared by intelligence agencies round the world."

The legislation has narrowed the number of agencies who are allowed to access metadata.

The list includes, among others, the Australian Federal Police, state police forces, Customs, the Australian Securities and Investments Commission (ASIC) and the Australian Competition and Consumer Commission (ACCC).

Now defined, other issues around how those agencies use and manage metadata will be debated, including the safety of the metadata, its vulnerability to criminal hacks, and how effective independent oversight of the scheme will be.

What is not indentified in the legislation is information about how metadata will be used and shared by Australian intelligence agencies and their Five Eyes partners - Canada, New Zealand, the United States and the United Kingdom.