Image caption US President Barack Obama has been pushing for a deal to be reached on the TPP

The Trans-Pacific Partnership (TPP) is one of the most ambitious free trade agreements ever attempted.

Its supporters have billed it as a pathway to unlock future growth of the countries involved in the pact.

The critics have been equally vociferous, not least because of the secrecy surrounding the negotiations of the agreement.

But despite the criticism, the countries involved have been pushing for a deal to be reached soon and they are confident that even more economies will want to join the pact in the coming years.

So what exactly is the TPP?

It is a proposed free trade deal currently being negotiated between 11 countries.

These are Australia, Brunei Darussalam, Canada, Chile, Malaysia, Mexico, New Zealand, Peru, United States, Singapore and Vietnam.

The pact is aimed at deepening economic ties between these nations.

It is expected to substantially reduce tariffs, and even eliminate them in some cases, between member countries and help open up trade in goods and services.

It is also expected to boost investment flows between the countries and further boost their economic growth.

The member countries are also looking to foster a closer relationship on economic policies and regulatory issues.

Media playback is unsupported on your device Media caption Watch: The BBC's Chief business correspondent, Linda Yueh explains whether President Obama's absence will hurt Trans-Pacific Partnership talks at the Apec summit.

What is the foundation of the TPP?

The 11 nations involved are looking to build up on a trade agreement that was originally signed between Brunei Darussalam, Chile, New Zealand and Singapore.

That agreement was called the Trans-Pacific Strategic Economic Partnership, or the P4, and came into effect in 2006.

It resulted in most tariffs on goods traded between these countries being removed immediately, with an agreement to gradually phase out remaining tariffs.

They also agreed to open government procurement contracts to businesses operating in any of the four countries.

The members of the P4 also said they will co-operate on issues such as customs procedures, labour practices, intellectual property and competition policies.

TPP: Nations currently negotiating Australia

Brunei Darussalam

Canada

Chile

Malaysia

Mexico

New Zealand

Peru

United States of America

Singapore

Vietnam

Why does TPP matter?

Well, it's all about numbers.

The 11 countries that are currently part of the negotiations are all members of the Asia-Pacific Economic Co-operation (Apec).

They have a combined population of more than 650 million people. A free trade agreement could turn this into a potential single market for many businesses.

The average per capita income in the participating countries was $31,491 in 2011 and their combined gross domestic product (GDP) stood at more than $20 trillion.

One cannot ignore the fact that the initiative is being led by the US, the world's biggest economy and biggest trading nation, and one that sees Asia-Pacific as key to its future growth.

Some analysts have even suggested that the US may be trying to use the TPP as a means to undermine China's growing economic might in the region.

Many believe that other members of the Apec bloc may also join the agreement in the coming years, making it an even more important pact.

In all, 21 Apec countries account for about 44% of global trade. They also make up some 40% of the world's population.

What is the status of the negotiations?

There are various dates during which talks were held at different levels.

However, it is safe to say that real negotiations of an expended treaty started only in 2010.

Since then, delegates and trade representatives have met for 16 rounds of discussion, focusing on a range of issues.

According to the US Trade Representative's (USTR) office, at the latest round of discussions held in Singapore this month, delegates "succeeded in finding solutions" to many issues in areas such as customs, telecommunications and regulatory coherence.

However, the USTR said that there are still challenging issues that need to be agreed on, including those related to intellectual property, competition and environment.

But there has been an increased push, not least from the US President Barack Obama, to finalise the pact soon.

The next round of TPP negotiations will be held in Lima, Peru, from 15 till 24 May.

Image caption There have been protests in Japan over the impact of TPP on its agriculture sector

Is there any criticism of the deal?

Yes, there is and it is on various fronts.

Like many other free trade agreements, there are fears over the impact TPP may have on certain products and services in member countries.

Some campaign groups have raised concerns about the impact such a wide-ranging agreement may have on intellectual property laws and patent enforcement.

The fear the deal may extend the scope of patents in sectors such a medicine and prevent the distribution of generic drugs.

Meanwhile Japan, which has expressed an interest to join the negotiations, has raised concerns about the agreement impacting its agriculture sector.

But the biggest criticism has been of what the campaigners allege to be secretive negotiations.

They say that the delegates have not been forthcoming about details of the issues that they have been discussing, and what the scope of agreement in those areas is likely to be, and how it will impact trade.

Last year, a group of lawyers even sent a letter to Ron Kirk, the US Trade Representative, to express what they called "profound concern and disappointment at the lack of public participation, transparency and open government processes in the negotiation of the intellectual property chapter of the TPP".

However, some analysts say the reason why the negotiations have not been made public is because there is no formal agreement on them as yet.

They also point out that free trade agreements generally attract a lot of criticism from campaign groups, and that in this case the delegates may be wanting to keep the discussions under wraps to avoid any pressure from such groups.