There are differing opinions in the cannabis world over what to call a group of plants that share certain physical, genetic, or chemical traits. While I'm usually more interested in being effective than being precise, I thought it might be useful to define some terms, and maybe uncover the source of some confusion.
We all agree that cannabis plants belongs to the scientific genus Cannabis. Some scientists believe there are distinct species of Cannabis (indica, sativa, ruderalis), while others believe there is only one species (sativa) with sub-types (subspecies or varieties). They can all interbreed, so the matter is primarily one of whether you are a taxonomic 'splitter' or 'lumper'.
Botanically speaking, a 'variety' is a sub-type of a species that has a certain trait in common with other members of that sub-type, and breeds true for that trait. If you believe there is only one species of Cannabis (sativa), you might also believe there is a variety within this species called Cannabis sativa var. indica. Or maybe it is a subspecies instead of a variety. Some organizations (agricultural ones) consider 'variety' and 'cultivar' to be interchangeable, but the botanical meaning is quite specific.
'Cultivar' is short for for 'cultivated variety'. Most of the plants in your garden are cultivars. They might be hybrids, they might be propagated by seed or by clone, and could be true-breeding or not. What is common between them is that people create and maintain a cultivar. Continuing our example, one might be growing Cannabis sativa var. indica 'Bubba Kush'. Cultivar is, in my view, the most proper term for cultivated cannabis types. However, as I noted before, 'cultivars' are often also referred to as 'varieties'. If you want to name a cultivar, there are several rules to follow.
At one time this word seems to have been used somewhat interchangeably with 'variety' 'breed' 'selection' or 'race' to indicate a low level sub-type. However, it is not a term used in botanical classification today, and is not a word I have encountered in the plant world outside of cannabis. I have run into it in biology, where a 'strain' is a sub-type of a microorganism (eg bacteria or virus) that has a particular genetic variant, such as influenza H1N1. In animal breeding, someone might refer to a particular breeding stock for a variety as a 'strain'... as in "Ben's strain of Barred Plymouth Rock chickens are heavier than Joe's", but in the vegetable breeding world, this type of difference would usually be referred to as a selection. This is true particularly when talking about older cultivars for which there may be many distinct types floating around, such as the 'Peckham' or 'Utah' selections from the 'Sweet Spanish' onion.
Another term frequently used in the cannabis community. Formally, a landrace is a cultivar (or variety) that has a specific geographical/historical origin where it developed (and to which it is presumably adapted). Landraces frequently contain quite a lot of genetic variation and have not be subjected to formal improvement processes. They are valued for their ability contribute specific traits to a breeding program (e.g. disease resistance) but are often too variable and unproductive for most cultivation purposes.
A genetic term pulled into common usage in the cannabis world, but not used commonly in other crops. Quite simply, the phenotype is the physical manifestation of an individual plant. Geneticists like to think about the phenotype as being a results of the combination of the effects of the genetic background of the organism (genotype) as influenced by the environment.
I hope this helps those who might have questions on this topic. If you wish to discuss plant improvement, genetics or breeding, I can be reached at firstname.lastname@example.org.