Ásatrú roughly translates to “true to the Aesir” or, colloquially, “belief in the [pre-Christian] gods,” and was coined by Icelandic Norse Pagans to give a name to old Icelandic Heathen practices. This corresponds to the founding of the Icelandic Ásatrúarfélagið in 1972. While inspired by old Norse practices, Ásatrú itself is a modern-day neo-pagan religion.
The term ‘Ásatrú’ (Anglicised as “Asatru”) has since acquired exclusive connotations in the US. The name is interpreted to suggest only the Aesir tribe gods should be the focus of deity veneration, which may or may not include the Vanir gods but typically excludes jötunn and other opposition to the Aesir. It’s also been co-opted by some American Folkish Heathen groups, so discernment is required when interpreting this word.