Heathrow airport

It's as simple as googling "heathrow airport meeting point" and looking for the (meeting point) symbol on the airport map for exact location

Be sure you know which terminal, though.

We have met up or been met at Heathrow several times. The easiest place is right outside the doors where passengers exit from the customs area ( which is just a walk-through) into the Arrivals Hall.

In T5 there is a barrier to deflect the passengers to the right as they exit,to prevent jam-ups at the doorway. People meeting incoming passengers often line up along the barrier or just a bit back, holding signs with the passengers' name if they do not know the person, or smiling and waving if they are meeting a friend. (Or, like my sister when she met us, they may hold a sign with the names even when they know the person well, just to be cute).

A imagine other terminals would have the same setup, but do not know from personal experience.

It is not the least bit chaotic or difficult, but it is a good plan to have your mobile phones enabled for calling or texting the UK just in case.

Do have a back up plan in case someone's flight is delayed. Are you planning to stay the night in London or are you immediately going on to somewhere else? If you are staying over in London, your plan could be that you will meet at the hotel (each person needs the hotel name/address and needs to know how to get transport in to the city) if someone is delayed.

As Norma said upthread, be sure to know which Terminal you are both arriving at. There are 4 terminals and they are spread out. Each of your airline websites should be able to tell you which terminal you are coming in to.

http://www.heathrow.com/airport-guide/getting-around-heathrow/airport-maps#

Five terminals and the Royal and VIP Suites. The terminals are not exactly close to each other, so not just a case of a quick walk if you get the wrong one. I always stand where the arrivals exit customs/air side, as do all the pre-booked private taxi's etc. Parking is very expensive if you're driving.

Better questions are: What you and your friend are intending to do after meeting? Are you traveling onward via an outbound flight from Heathrow? Are you meeting up for a stay in London? Are you arriving at Heathrow on a flight or are you arriving via ground transportation?

If you are already in London and you and your acquaintance are staying in London, I would arrange to meet them at the tube station in the terminal which their flight is arriving. This information can easily be found on Heathrow's web site.

If you are each traveling onward, I would meet at a specified point in the terminal your flight is departing from.

Heathrow Airport is one of the busiest Airport in London, It has 5 terminals. Before going to meet your friend in Heathrow first of all decide which terminal is the best to meet.
Hope you will plan Lunch with your friend so checkout the Hotels nearby Heathrow airport, For your time save here i'm listing the Hotels name,
a) Giraffe
b) Carluccio's
c) Caffè Nero
d) Caviar House & Prunier Seafood Bar

After Lunch, have a some entertainment to your friend by visiting attraction places near to Heathrow Airport,
a) Hounslow Urban Farm
b) National Trust - Claremont Landscape Garden
c) LEGOLAND Windsor Resort

If you have more time visit few more places through around London i.e.. London eye, Winter Wonderland, Duck Tours, etc..

If you are planning to visit your friend on Christmas or New Year day, make sure all the things are opened on that day or not and plan your meet up.