Does a monk's Step of the Wind ki feature affect High Jumps as well, or just Long Jumps?

At 2nd level, monks can spend a ki point to use the Step of the Wind option of the Ki feature (emphasis mine):

You can spend 1 ki point to take the Disengage or Dash action as a bonus action on your turn, and your jump distance is doubled for the turn.

What is meant by "jump distance", exactly?

The rules on Jumping state (bold italics emphasis mine):

Long Jump. When you make a long jump, you cover a number of feet up to your Strength score if you move at least 10 feet on foot immediately before the jump. When you make a standing long jump, you can leap only half that distance. Either way, each foot you clear on the jump costs a foot of movement. [...]

High Jump. When you make a high jump, you leap into the air a number of feet equal to 3 + your Strength modifier (minimum of 0 feet) if you move at least 10 feet on foot immediately before the jump. When you make a standing high jump, you can jump only half that distance. Either way, each foot you clear on the jump costs a foot of movement. [...]

You can extend your arms half your height above yourself during the jump. Thus, you can reach above you a distance equal to the height of the jump plus 1 1/2 times your height.

Both Long Jump and High Jump mention distance, so does that mean a monk who uses Step of the Wind can jump twice as high? I ask because my DM and other players in the group who spoke up are under the impression it only affects the Long Jump, and that a High Jump is unaffected by Step of the Wind...

The section on "Jumping" lumps both forms together

Your Strength determines how far you can jump. [...]

It then describes both long jump and high jump, so we can conclude that "how far you can jump" refers to both types of jumping. This also matches a standard English reading.

When a feature benefits a certain type of jumping it specifies as such

• The control winds spell:

[...] When a creature in the cube makes a vertical jump, the creature can jump up to 10 feet higher than normal.

• The Totem Warrior Barbarian's 3rd level Tiger Totem feature

While raging, you can add 10 feet to your long jump distance and 3 feet to your high jump distance. [...]

• The Champion Fighter's Remarkable Athlete feature:

[...] when you make a running long jump, the distance you can cover increases by a number of feet equal to your Strength modifier.

Notably, the Tiger Totem feature even calls it a "high jump distance" supporting the idea that your "jump distance" would refer to both forms of jumping.

Various features change jump distance

• The Step of the Wind feature

[...] your jump distance is doubled for the turn.

• The jump spell

[...] The creature's jump distance is tripled until the spell ends.

• The Boots of Striding and Springing magic item

[...] you can jump three times the normal distance [...]

We can conclude from all these that when something increases your "jump distance" it increases both your high jump and your long jump distances. If something were meant to affect only long jump or only high jump, it would have stated that similar to the Tiger Totem feature and control winds spell.

A final piece of evidence is the Thief Rogue's Second-Story Work feature:

[...] when you make a running jump, the distance you cover increases by a number of feet equal to your Dexterity modifier.

I would assume that this feature benefits any running jump, not just a running long jump like Remarkable Athlete. I think this is the best evidence that when the rules want to refer specifically to one style of jumping they will do so.

Yes, it affects both

The language used in the PHB and other books is very particular. Writing your "jump distance is doubled" and not "your long jump distance is doubled" would be intentional. Thus, anything that affects your "jump distance" in general affects both high jumps and long jumps.

• Maybe a useful comparison to make that "increasing your speed by 10" applies to all speeds be that walking, flying, swimming, or borrowing. Commented May 23, 2020 at 19:33