Evergreen

The term "evergreen", as in the context of "an evergreen keyword", refers to keywords or keyword actions that can be used in any set.

As the core set has always been the place for new players to start, as of Eighth Edition it was decided to include reminder text with the evergreen creature keywords.

Primary/secondary/tertiary
When talking about which colors get which evergreen creature keywords, R&D tends to talk about a system called "primary/secondary/tertiary". In their quest to differentiate the colors in the color wheel, each should have strengths and weaknesses. Therefore, not every color gets every keyword.
 * Primary This is the color (or colors) the ability is seen most in. That means it shows up in the highest volume and usually at the lowest rarity that the type of effects get used at. The primary color will almost always get this effect in a set if it's an ability we do every set. It also tends to be the color that most often pushes the power level, if it's an effect we push the power level on. There's a wide range on what primary means, because different types of effects exist at different levels. A color secondary in flying can show up way more than a color primary in taking extra turns, for instance, because we have so many more flying cards than extra-turn cards.
 * Secondary This is the color (or colors) that an ability shows up in on a somewhat regular basis, but not as often as the primary and not always in as low of rarity as the primary. If the effect is something we do a lot of, the secondary color will usually get the ability in most sets. Sometimes a secondary color will have restrictions. For instance, red is secondary in flying, but only on Dragons and Phoenixes.
 * Tertiary This is the color (or colors) that get the ability occasionally. It's not every set. For some abilities, we could go years without seeing a tertiary color using it. Tertiary colors can often come with rules, meaning it's a very narrow subset that makes use of the mechanic. For example, black is tertiary in first strike and gets it primarily on Knights, most often when paired against a mirrored white Knight.

Former evergreen keywords
Most these have been obsoleted, but protection was considered to be deciduous. It is a tool in R&D's toolbox that they're allowed to use, but it's not something they expect to use in every set.

However, protection was reinstated as Evergreen in Core Set 2020.

Evergreen keywords by color
Of particular note is the evergreen creature keywords each shared between two colors. Prowess formerly served as the red/blue keyword before it lost its evergreen status.