a pillow that is often put across a bed underneath the regular pillows
"She placed an extra bolster at the foot of the bed to provide comfort while reading in the evening."
A large cushion or pillow.
"She placed a soft bolster at her head to prop herself up while reading in bed."
prop up with a pillow or bolster
"She rolled over and placed a thick cushion behind her back to bolster herself while reading in bed."
To brace, reinforce, secure, or support.
"The experienced carpenter used a heavy wooden beam to bolster the wobbly ladder before climbing up."
A surname.
"The local historian wrote a biography about Mr. Bolster, whose family has lived on the farm for five generations."