SegWit was activated on the Bitcoin network at block height 481824.
SegWit, short for Segregated Witness, was a Bitcoin soft fork activated on 24 August 2017. It changed how transaction data and signature data were organized, addressing transaction malleability and increasing effective block capacity without simply raising the traditional one-megabyte block-size limit.
The upgrade used miner signaling during 2,016-block difficulty periods. After the required support threshold was reached, SegWit locked in and then activated at the start of the next signaling period. That staged process is why lock-in and activation are sometimes reported as separate dates or heights.
A frequent mix-up is with Bitcoin Cash’s fork at block 478558, which happened earlier in August 2017. SegWit did not create a permanent Bitcoin chain split; it was a backward-compatible soft fork. Its malleability fix also helped make technologies such as the Lightning Network practical.