The 'breakout' is the moment they stop being considered simply as 'one for the future' and become an important part of the present.
In that sense, Eden Hazard had his 'breakout season' many years ago during his time with Lille. As a teenager, the Belgian had been a regular part of a good team, but upon turning 20 years of age he reached a level and has been on an almost exclusively upward trajectory ever since.
That 'breakout season' came in 2011/12, one year after Lille completed a league and cup double in France, scoring 22 goals in all competitions and arguably becoming world class from then on.
But after an excellent World Cup in which he captained Belgium to a best ever third place finish and a blistering start to the new 2018/19 Premier League season with five goals, two assists and five wins in the opening five games, is Hazard ready for another 'breakout'?
Despite his undisputed world class status, Hazard has never yet cracked the very top bracket of elite global players where Cristiano Ronaldo and Lionel Messi perform, joined by the likes of Neymar, Xavi, Andres Iniesta, Antoine Griezmann, Franck Ribery, Arjen Robben, Gareth Bale and other superstars at various times over the course of the last decade.
For Hazard, the chance to make that leap to the highest level of his profession is now.
One complaint of the Belgian, who grew up idolising Zinedine Zidane, has long been that he never manages to score enough goals given his level of influence and talent. Hazard hasn't scored 20 goals in a single campaign since his final year at Lille, an annual milestone that Stamford Bridge predecessor Frank Lampard hit on a regular basis.
His number of assists per season has also generally fallen at Chelsea, from 14 in the Premier League in 2012/13 to four in 2017/18.
But having started so well this season, Chelsea manager Maurizio Sarri has great faith in Hazard to reach new heights, even backing him to win the Premier League Golden Boot - an award very rarely won by players who aren't considered traditional strikers.
"I think [Hazard can win the Premier League Golden Boot]. We have spoken and I told him he can score 40 goals. He has to improve some things, but he can do it," the Italian is quoted as saying by Sky Sports after Hazard scored a hat-trick against Cardiff over the weekend.
In comments made on BBC Match of the Day, Sarri said: "Hazard is unique. I thought he was one of the best in Europe. But now I am changing my mind as maybe now he is the best in Europe."
At the age of 27, Hazard is arguably now entering his peak. If he is ever going to be among the true elite, he has to take the next step. Everything is in place for that to happen, the player just needs to make 2018/19 into the second 'breakout season' of his career.