In the normal course of events, implementations of 'tcpdump' report on packets going in both directions, which is to say it reports both packets received and packets sent. Normally this isn't confusing and you can readily tell one from the other, but sometimes situations aren't normal and you want to see only incoming packets or only outgoing packets (this has come up before). Modern versions of tcpdump can do this, but you have to know where to look.

If you're monitoring regular network interfaces on Linux, FreeBSD, or OpenBSD, this behavior is controlled by a tcpdump command line switch. On modern Linux and on FreeBSD, this is '-Q in' or '-Q out', as covered in the Linux manpage and the FreeBSD manpage. On OpenBSD, you use a different command line switch, '-D in' or '-D out', per the OpenBSD manpage.

(The Linux and FreeBSD tcpdump use '-D' to mean 'list all interfaces'.)

There are network types where the in or out direction can be matched by tcpdump pcap filter rules, but plain Ethernet is not one of them. This implies that you can't write a pcap filter rule that matches some packets only inbound and some packets only outbound at the same time; instead you have to run two tcpdumps.

If you have a (software) bridge interface or bridged collection of interfaces, as far as I know on both OpenBSD and FreeBSD the 'in' and 'out' directions on the underlying physical interfaces work the way you expect. Which is to say, if you have ix0 and ix1 bridged together as bridge0, 'tcpdump -Q in -i ix0' shows packets that ix0 is receiving from the physical network and doesn't include packets forward out through ix0 by the bridge interface (which in some sense you could say are 'sent' to ix0 by the bridge).

The PF packet filter system on both OpenBSD and FreeBSD can log packets to a special network interface, normally 'pflog0'. When you tcpdump this interface, both OpenBSD and FreeBSD accept an 'on <interface>' (which these days is a synonym for 'ifname <interface>') clause in pcap filters, which I believe means that the packet was received on the specific interface (per my entry on various filtering options for OpenBSD). Both also have 'inbound' and 'outbound', which I believe match based on whether the particular PF rule that caused them to match was an 'in' or an 'out' rule.

(See the OpenBSD pcap-filter and the FreeBSD pcap-filter manual pages.)