A rachis is the central stem of a compound leaf, the axis its leaflets branch from. Look at an ash leaf, a rowan or the frond of a fern. The long stalk down the middle is the rachis, and the leaflets fan out along it.
The petiole and the midrib get muddled with it. The petiole is the stalk joining the whole leaf to the stem, and the rachis carries on from the point where the lowest pair of leaflets sits. The midrib is the central vein inside a single blade, whether that blade is a simple leaf or one leaflet of a compound one. The plural is rachises, or rachides if you want the Greek form.
Botanists use the word beyond compound leaves. In a grass, or in any spike of flowers, the rachis is the main axis the florets sit along. In a fern it runs through the blade, above the bare stalk at the base.
Raychis is a deliberate misspelling of it. People keep asking how to say it: RAY-kiss.
We chose it because species identification sits at the centre of the app. Once Raychis knows the species, it can give better care advice, health checks and seasonal guidance. If the species is wrong, the rest of the advice is wrong too.
A lot of plant apps stop at identification. Raychis uses identification as the starting point.
You can see the idea worked through in the plant guides, where every entry starts from the species and opens out into what it needs and what tends to go wrong with it. Ash, rowan, elder and wisteria all carry a rachis, if you want to go and find one.