This was easy to find because pg 276 temporary hit points glossary entry says: "Temporary Hit Points are granted by certain effects and act as a buffer against losing real Hit Points. See also chapter 1 ("Damage and Healing"). "I see nothing in the 5.5 spell description that would prevent it from stacking.
PHB29 damage and healing section has this to say about them.
TEMPORARY HIT POINTS
Some spells and other effects confer Temporary Hit
Points, which are a buffer against losing actual Hit
Points, as explained below.
LOSE TEMPORARY HIT POINTS FIRST
If you have Temporary Hit Points and take damage,
those points are lost first, and any leftover damage
carries over to your Hit Points. For example, if you
have 5 Temporary Hit Points and take 7 damage,
you lose those points and then lose 2 Hit Points.
DURATION
Temporary Hit Points last until they're depleted or
you finish a Long Rest (see the rules glossary).
THEY DON'T STACK
Temporary Hit Points can't be added together. If
you have Temporary Hit Points and receive more
of them, you decide whether to keep the ones you
have or to gain the new ones. For example, if a spell
grants you 12 Temporary Hit Points when you al-
ready have 10, you can have 12 or 10, not 22.
THEY'RE NOT HIT POINTS OR HEALING
Temporary Hit Points can't be added to your Hit
Points, healing can't restore them, and receiving
Temporary Hit Points doesn't count as healing.
Because Temporary Hit Points aren't Hit Points, a
creature can be at full Hit Points and receive Tem-
porary Hit Points.
If you have O Hit Points, receiving Temporary Hit
Points doesn't restore you to consciousness. Only
true healing can save you.
Some spells and other effects confer Temporary Hit
Points, which are a buffer against losing actual Hit
Points, as explained below.
LOSE TEMPORARY HIT POINTS FIRST
If you have Temporary Hit Points and take damage,
those points are lost first, and any leftover damage
carries over to your Hit Points. For example, if you
have 5 Temporary Hit Points and take 7 damage,
you lose those points and then lose 2 Hit Points.
DURATION
Temporary Hit Points last until they're depleted or
you finish a Long Rest (see the rules glossary).
THEY DON'T STACK
Temporary Hit Points can't be added together. If
you have Temporary Hit Points and receive more
of them, you decide whether to keep the ones you
have or to gain the new ones. For example, if a spell
grants you 12 Temporary Hit Points when you al-
ready have 10, you can have 12 or 10, not 22.
THEY'RE NOT HIT POINTS OR HEALING
Temporary Hit Points can't be added to your Hit
Points, healing can't restore them, and receiving
Temporary Hit Points doesn't count as healing.
Because Temporary Hit Points aren't Hit Points, a
creature can be at full Hit Points and receive Tem-
porary Hit Points.
If you have O Hit Points, receiving Temporary Hit
Points doesn't restore you to consciousness. Only
true healing can save you.
This was of course also how temporary hit point stacking worked in 5.0 and the nonstacking was explained in the 5.0 phb on page 198 where you had the damage and healing section's relevant bit. Good on 5.5 for demonstrating why a well constructed glossary is a tool that supports the GM during play by making this sort of rules reference into a quick & trivial thing that can be tossed back on the player trying to make a case based on poor selective reading of the rules with a "what does the glossary say about [thing]"







