These are some definitions used in this guide:

IBC transactionA transaction that includes IBC messages (including packets). This is constructed by the relayer and sent over the physical network to a chain according to the chain rules. For example, for tendermint chains a broadcast_tx_commit request is sent to a tendermint RPC server.
IBC messageAn element of the transaction payload sent by the relayer; it includes client, connection, channel and IBC packet data. Multiple IBC messages may be included in an IBC transaction.
IBC packetA particular type of IBC message that includes the application packet and its commitment proof.
IBC ClientClient code running on chain, typically only the light client verification related functionality.
Relayer Light ClientFull light client functionality, including connecting to at least one provider (full node), storing and verifying headers, etc.
Source chainThe chain from which the relayer reads data to fill an IBC message.
Destination chainThe chain where the relayer submits transactions that include the IBC message.