Subcutaneous
Subcutaneous means into the layer of fatty tissue directly beneath the skin, rather than into a muscle or a vein.
The layer has a comparatively modest blood supply, so material introduced there tends to be absorbed gradually rather than all at once.
It is one of several routes a product's labelling might specify, and which route applies to any given product is a property of that product, not something to be inferred from the category it belongs to.
What it is not
Subcutaneous is not intramuscular and not intravenous. These are different routes with different absorption and different risks, and they are not substitutes for one another.