What is Essential Travel?

First off, the advisory that you linked is a travel advisory. This means that it is just that: Advice that you should not currently travel without an important reason.

As there is no prohibition, you don't need an exact definition, as everyone makes the decision for themselves.

Let's look at the types of travel restrictions that you can currently face:

Travel advisories

Almost any government on the planet currently has a travel advisory similar to the Canadian one. A travel advisory means that the government strongly suggests that you shouldn't travel, but they won't stop you if you decide otherwise.

For many bookings a travel advisory is also valid grounds for a cancellation. Also, it will be virtually impossible to buy insurance (including travel health insurance) after the advisory is in effect.

Non-essential travel in this context means mainly tourism, but could also extend to business trips that you can cancel without negative repercussions.

Travel prohibitions, outgoing

It extremely rare for a western country to prohibit their citizens to travel. An example is the travel restriction for North Korea that the US imposes on their citizens. Even in this case, they do not actively seek to stop visits, but simply declare their passports invalid for travel.

I know of no western country that currently prevents their citizens from leaving due to COVID-19.

However, some countries in Africa (like the DR Congo) have put a complete lockdown in place, cancelled all incoming and outgoing flights and banned all travel -- this affects their own citizens as well as foreigners.

Entry restrictions

A growing number of countries has restrictions on who may enter. Some do no longer allow travellers from areas with active outbreaks, others (like Germany) have closed their borders to all travellers without an "important reason".

Again, I know of no western democracies who prevent their own citizens from entering. However, some have enacted restrictions for arrivals (e.g. Canada apparently disallows you to arrive by air with COVID-19 symptoms, and other countries may force you into quarantine if you arrive from certain areas).

Some countries apply their restrictions even to transit passengers, which makes travel even more difficult.

If a country bans entry, the airline will not allow you to board unless you can demonstrate that you'll be let in.

Internal restrictions

Some countries have restricted domestic travel. In France and Italy you may only leave your home for urgent reasons, and internal travel is essentially prohibited unless you meet one of the exception criteria.

These countries still allow their own citizens to enter and travel to their residence, but will not allow touristic travel by foreigners.

Practical restrictions

Most affected areas have heavily restricted public life. Most shops are closed, and hotels are often not open for tourist visitors.

And, even if you are legally allow to travel somewhere, many flights are cancelled and public transport may operate on a reduced schedule or be suspended.

Conclusion

Generally speaking, there is no easy answer to your question, as it all depends on where you come from and where you go.

In general, it is currently a good idea to not travel at all.

Even though the Canadian government will allow you to leave, there are many destinations that you can't reach and you may easily get into a situation where it is difficult, expensive or outright impossible to get back home.

The definition is still quite open ended in Canada; use your own judgment, listen to local public health recommendations and obey them.

(AFAIK) Each province sets its own travel restriction, in Québec, my place of residence, it is highly advised not to travel between regions.

From today (23h59) every non-essential businesses are closed, and corollary, a list of essential business allowed to stay open; so everything else is closed, so that limit the need to get out of home to travel (locally or regionally).

Business meetings can and should be be done by phone/video-conferences, factory visits should be postponed.