Without any real physical form or material existence.
Something substanceless might seem like it's not really there or not important.
Examples:
The ghost was substanceless, so I could see right through it.
Her promise felt substanceless, like it wasn't real.
The idea of a substanceless cloud seemed strange to me.
The substanceless cake looked real, but it wasn't.
The substanceless mist made everything look strange and blurry.
The substanceless dream felt like it lasted forever.