Askur is an ash-tree, fraxinus, which belongs to Yggdrasill.
So Yggdrasill is not a tree, but the owner of the ash.

Often, though, the tree itself is called Yggdrasill, even if the tree is not all too correctly termed so
askur Yggdrasils, now what is ygg, what is dras?
Yggur is a name of Óðinn, and drasill, drösull, a horse
but mind you:  dras dris could just as well by the all-seeing, Sanskrit pashyanti, the clairvoyance of the divine one
drösull, tras त्रस्  “seeing”; दृष् , drís ashwa stha, अश्वत्त
askurinn (the ash) is mjötviður mær, the precious tree which sets limits, mjöt, the appropriate amount, and marks the fate of our universe

mjötuður is fate, örlög, end of our universe (big rip or big crunch).

At ragnarök ymur hið aldna tré, the olden tree moans, as its time has come to an end.
No man knows the origin of the 3 roots  –  manngi veit hvers af rótum renn

to our askur belong
Heiðrún the goat,
Ratatoskur the squirrel
Níðhöggur the dragon/worm who gnaws the roots
and the 4 stags

but the tree Læraður is in Valhöll

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Listen to pronunciation: askur Yggdrasils; ragnarök