Suricata

I would say 'flow.active' or 'flow.current' or something.

Btw there were a good bunch of counters added in git master already just a few days ago -
https://github.com/inliniac/suricata/commit/70c16f50e733f6f7cc40c1bc3465eb966e3be517

flow_mgr.flows_checked: number of flows checked for timeout in the last pass

flow_mgr.flows_notimeout: number of flows out of flow_mgr.flows_checked that didn't time out

flow_mgr.flows_timeout: number of out of flow_mgr.flows_checked that did reach the time out

flow_mgr.flows_removed: number of flows out of flow_mgr.flows_timeout that were really removed

flow_mgr.flows_timeout_inuse: number of flows out of flow_mgr.flows_timeout that were still in use or needed work

flow_mgr.rows_checked: hash table rows checked
flow_mgr.rows_skipped: hash table rows skipped because non of the flows would time out anyway

The counters below are only relating to rows that were not skipped.

flow_mgr.rows_empty:   empty hash rows
flow_mgr.rows_maxlen:  max number of flows per hash row. Best to keep low, so increase hash-size if needed.
flow_mgr.rows_busy:    row skipped because it was locked by another thread